Eliminate potential nondeterminism when printing sample labels,
which are internally represnted as a map.

Should have no functional changes

- Remove dead code, unnecessary conversions
- Consistently use a single space after period

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.

Also update the build instructions to use go get