Using generic fb_serialize/fb_unserialize implementation

Using generic template-based fb_serialize/fb_unserialize implementation in HPHP.
Removing errcode from fb_unserialize API, since it's never used.

Reviewed By: alikhtarov

Differential Revision: D959470
Use camelCase for namespace functions

Style fix.  Only class static methods should use
initial caps.

The few callsites we have don't need to be changed since they're
using the HHVM_FE/HHVM_ME macros which this diff also changes.

Reviewed By: @jdelong

Differential Revision: D965366