Add tests for CheckValid, fix a couple crashers.

* Add support for tag units

Updated tests for adding tag units

* updated proto docs to describe tag units

* fixed formatting

* added additional test

* require specific units for graph tag to be recognixed

* updated tests and formatting

* Fixed formatting

* Fixed style error

* modify proto_test to be sure tag units not overwritten

* Clarified label docs

* call legacy profile memory allocations size, not byte

* updated tests to call bytes tags in legacy profile size

* Revert "updated tests to call bytes tags in legacy profile size"

This reverts commit 9289378af2.

* Revert "call legacy profile memory allocations size, not byte"

This reverts commit 6b18973562.

* Updated so units not modified when profile written out

* Fixed formatting errors

* Removed field for inferred numeric label units from profile

* Modified profile String() output

* fixed formatting error

* changed name of function used to identify units for numeric labels

* Refactor String() for profile

* Modified for clarity and address comments

* renamed function for clarity

* Made numeric label units field seperate in profile

* Modified test

* Refactored identifyNumLabelUnits()

* Updated comments

* Fixed formatting error

* Addressed comments

* Fixed style error

* clarify comment

* Refactored to address comments

* addressed comments

* addressed comments

* addressed comments

Added some multi-threaded tests that allow the race detector
to detect race conditions in pprof internals.

Do so by relaxing the matching to treat all mappings for file names
starting with a '[' as not symbolizable.

Trying to symbolize files like /dev/dri/renderD128 makes pprof hang as
it tries to read from this device file. So do not attempt the symbolization.

Skip [heap] file, too - the profile from issue #78 has a couple samples
in the heap (perhaps some generated code) and trying to locate a file
with that name can't succeed.

Fixes #78.

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