Often profile handlers return no data, and pprof currently
returns "profile format unrecognized", which confuses users
and sends them on a wild goose chase.

Print a more explicit "empty input file" error message.

The local symbolization already skips these mappings. Have the helper
which can be used when alternative symbolization flows are used.

Some profile handlers will split mappings into multiple adjacent
entries. This interferes with pprof when overriding the main binary.

Currently there is code that attempts to combine these mappings, but
it fails if the mapping offsets aren't available. Add a check to
combine them in that situation.

By default hide entries in the comment field that starts with #, and
add a "comments" command to show all comments.

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