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