The names produced by addr2line are often incomplete.  Fix this by
falling back to nm to get function names and using the nm-provided
name if it is longer.

When reconciling samples, cleanup file paths to allow functions to be
combined if their source filenames match after cleanup (eg "src.cpp" and
"./src.cpp").

Also clean up existing uses of filepath to avoid passing empty strings
to it: filepath.Base("") returns "."

internal/driver: fix spelling in help text

Add field for profile sources to indicate what is the primary
sample value, which visualizers should display by default.
Previously there was a convention for pprof to use the last
sample_index by default, this provides more flexibility.

Instead of including it the demangle package with pprof,
point to its own repo.

remove binutils wrappers used for testing

Provide native Go mocks for binutils to use during testing, instead of
relying on shell-script mocks of those tools.

Also update the build instructions to use go get