Various internal improvements

Mostly related to keeping piqic-erlang and piqic-ocaml in sync.
piqic-erlang: fix generation of incorrect code for custom Erlang types

The fix is ported from piqic-ocaml.
Fix multi-format serialization of built-in types
Improve code that generates Erlang types
Add .rebar to gitignore
Get rid of Makefile.piqi files