[cmx] Add cmx, a tool for cmx processing logic

This tool can format, validate, and merge CMX files. Add build rules
that perform these actions as appropriate. Add //build/rustfmt.toml to
support rustfmt'ing code in //build.

CP-85 #comment
CP-86 #done
CP-119 #done

Change-Id: Ibd9f3f76a6cd4279b583d8205ebceed7fd0722e4
13 files changed