Clean up what we've declared "//visibility:public" in the build files.

[garretrieger]

• I've reviewed the list of header files that have public visibility and removed it in a few cases that are more implementation details (for example segmentation context and the TRY macros).
• Added a script which checks that all includes from a public header are themselves public headers and fixed some cases where this wasn't the case.
• Dropped all of the "IFTB" stuff as it's not used anymore, now that we have a segmenter implemented in this repo.
• Added name spacing to the various proto messages.

[nyshadhr9]

LGTM