Add symbolization support for Mach-O

Fix tree trimming

This will enable symbolization support for Go on Mac OS
It re-enables symbolization using debug/pprof/symbol on
Go profiles in the legacy format, and implements basic
mach-O support on the binutils package.

This is to keep the new TrimTree functionality from breaking any code
currently using the public interface. We do this by separating NodeSet
from nodePtrSet and creating different functions for each.

If we are deleting a node N, if the edge between N and its parent and
the edge between N and its child are both inline, then the resulting
residual edge from N's parent and the child will also be inline.

In a graph, NodeInfo maps one to one to the nodes, so it suffices to
just find the top NodeInfo s and only keep those nodes in the graph. In
a tree however, a single NodeInfo may map to many nodes. As of this
commit, a call to 'web 10' in pprof on a tree will return all the nodes
corresponding with the top 10 NodeInfo s.

Makes the report label accurate

Removing this file does not break any tests, though removing other cppbench.*
files do.

It is no longer representative of how omissions are made. pprof uses
more than just the cumulative value to make omissions.

Before this change, the node count used in the label is the proposed amount
provided by the user. If some nodes were trimmed and the graph ended up with
less nodes than the user asked for, the report label will now reflect this.

Allow weblist to work even if assembly is not available

Weblist provides source and assembly combined in a web document.
It needs access to the binary to print the assembly, but currently
refuses to generate the source if the binary can't be find.

Fall back to just generating the source if the binary isn't found.

Split source filepath cleanup from file opening

Reports want to use trimmed paths, not the location in the current
filesystem which may be very verbose.

Add mechanism for extensions to set variable defaults

This enables pprof extensions to update the default for a variable
before the command line is processed.

Add source_path option to point pprof to source files

Currently pprof will look for source files only on the current directory
and its parents. This makes it hard to examine sources on jobs where
there are multiple source trees (eg from different libraries).

Add a variable to provide a search path for source files. It will default
to the cwd, so there will be no change in behavior by default.

Generate kcachegrind reports under line granularity

Callgrind reports are generated with line granularity since kcachegrind
can take advantage of the information. However, kcachegrind reports were
not being generated with that granularity, creating an unnecessary difference.

Make Node sorting check all Info fields

Graphs are built using Go maps which do not provide a determinist
traversal. We sort the nodes to remove the non-determinism, but
the sort fails to provide a fully deterministic ordering in cases
where the attributes being sorted have equal values. Detect those
cases and fallback to a full comparison of all fields.

Commit d89f0ee updated some verification files and
removed some whitespace that is needed for verification.

Have peek command honor "tagfocus"

The peek command allows looking at details of a function, and it
avoids any trimming to provide full details. However, when using
tagfocus it is useful to still limit to the filtered samples.
With this change peek will honor tagfocus, but it will avoid any
other trimming.

Apply -1 adjustment to leaf frames for some legacy profiles

Legacy profiles that are not based on interrupt-based sampling
only include addresses that are call stack return addresses. For
them, all callstack addresses should be adjusted by -1 to land
on top of the call instruction and permit accurate symbolization.

Ignore internal fields when comparing value types

This reverts commit 9c191ebbce.
The trimproto option is no longer needed, removing it to avoid code growth.

Extend regexp for matching legacy growth profiles

This fixes issue #9