Callgrind import format support (#331)

Implements import from the [callgrind format](https://www.valgrind.org/docs/manual/cl-format.html).

This comes with a big caveat that the call graph information contained with callgrind formatted files don't uniquely define a flamegraph, so the generated flamegraph is a best-effort guess. Here's the comment from the top of the main file for the callgrind importer with an examplataion:

```
// https://www.valgrind.org/docs/manual/cl-format.html
//
// Larger example files can be found by searching on github:
// https://github.com/search?q=cfn%3D&type=code
//
// Converting callgrind files into flamegraphs is challenging because callgrind
// formatted profiles contain call graphs with weighted nodes and edges, and
// such a weighted call graph does not uniquely define a flamegraph.
//
// Consider a program that looks like this:
//
//    // example.js
//    function backup(read) {
//      if (read) {
//        read()
//      } else {
//        write()
//      }
//    }
//
//    function start() {
//       backup(true)
//    }
//
//    function end() {
//       backup(false)
//    }
//
//    start()
//    end()
//
// Profiling this program might result in a profile that looks like the
// following flame graph defined in Brendan Gregg's plaintext format:
//
//    start;backup;read 4
//    end;backup;write 4
//
// When we convert this execution into a call-graph, we get the following:
//
//      +------------------+     +---------------+
//      | start (self: 0)  |     | end (self: 0) |
//      +------------------+     +---------------|
//                   \               /
//        (total: 4)  \             / (total: 4)
//                     v           v
//                 +------------------+
//                 | backup (self: 0) |
//                 +------------------+
//                    /            \
//       (total: 4)  /              \ (total: 4)
//                  v                v
//      +----------------+      +-----------------+
//      | read (self: 4) |      | write (self: 4) |
//      +----------------+      +-----------------+
//
// In the process of the conversion, we've lost information about the ratio of
// time spent in read v.s. write in the start call v.s. the end call. The
// following flame graph would yield the exact same call-graph, and therefore
// the exact sample call-grind formatted profile:
//
//    start;backup;read 3
//    start;backup;write 1
//    end;backup;read 1
//    end;backup;write 3
//
// This is unfortunate, since it means we can't produce a flamegraph that isn't
// potentially lying about the what the actual execution behavior was. To
// produce a flamegraph at all from the call graph representation, we have to
// decide how much weight each sub-call should have. Given that we know the
// total weight of each node, we'll make the incorrect assumption that every
// invocation of a function will have the average distribution of costs among
// the sub-function invocations. In the example given, this means we assume that
// every invocation of backup() is assumed to spend half its time in read() and
// half its time in write().
//
// So the flamegraph we'll produce from the given call-graph will actually be:
//
//    start;backup;read 2
//    start;backup;write 2
//    end;backup;read 2
//    end;backup;write 2
//
// A particularly bad consequence is that the resulting flamegraph will suggest
// that there was at some point a call stack that looked like
// strat;backup;write, even though that never happened in the real program
// execution.
```

Fixes #18

sample/profiles/callgrind/callgrind.example.log

@@ -0,0 +1,24 @@
+# callgrind format
+events: Instructions
+
+fl=file1.c
+fn=main
+16 20
+cfn=func1
+calls=1 50
+16 400
+cfi=file2.c
+cfn=func2
+calls=3 20
+16 400
+
+fn=func1
+51 100
+cfi=file2.c
+cfn=func2
+calls=2 20
+51 300
+
+fl=file2.c
+fn=func2
+20 700

sample/profiles/callgrind/callgrind.multiple-event-types.log

@@ -0,0 +1,29 @@
+version: 1
+creator: xdebug 3.0.2 (PHP 7.4.14)
+cmd: /var/www/html/index.php
+part: 1
+positions: line
+
+events: Time_(10ns) Memory_(bytes)
+
+fl=(1) file1.c
+fn=(1) main
+16 20 15000
+cfn=(2) func1
+calls=1 50
+16 400 20000
+cfi=(2) file2.c
+cfn=(3) func2
+calls=3 51
+16 400 3000
+
+fn=(2)
+51 100 4000
+cfi=(2)
+cfn=(3)
+calls=2 20
+51 300 5000
+
+fl=(2)
+fn=(3)
+20 700 6000

sample/profiles/callgrind/callgrind.name-compression.log

@@ -0,0 +1,24 @@
+# callgrind format
+events: Instructions
+
+fl=(1) file1.c
+fn=(1) main
+16 20
+cfn=(2) func1
+calls=1 50
+16 400
+cfi=(2) file2.c
+cfn=(3) func2
+calls=3 20
+16 400
+
+fn=(2)
+51 100
+cfi=(2)
+cfn=(3)
+calls=2 20
+51 300
+
+fl=(2)
+fn=(3)
+20 700