compiler: do not use symbols with beginning with __

Symbols beginning with double underscore are in the namespace of the
compiler.  Rename:

* __fi_allocinfo --> fi_allocinfo_internal
* __fi_freeinfo --> fi_freeinfo_internal
* __fi_datatype_size --> fi_datatype_size_table
Merge pull request #186 from jsquyres/topic/compiler-namespace-fixes

compiler: do not use symbols with beginning with __