Major API update

This release contains many changes to increase consistency, flexibility, and usability:

• addConst is now addConstant
  • if called on a class, it now takes a CodegenClassConstant object
  • if called on a file, it now takes a CodegenConstant object
• addTypeConst is now addTypeConstant and takes a CodegenTypeConstant object
• adds CodegenShape and CodegenShapeMember objects
• adds HackBuilderValues::codegen() to allow building a Codegen* as a value
• renamed various *Base classes to *ish for consistency with other internal classes, HHAST, and the Hack parser - for example, CodegenClassish
• adds support for adding constraints to generics
• uses HSL format strings throughout
• uses \0 instead of \t as the separator for functions that accept suggested whitespace, allowing use where tabs are desired instead of spaces
• add HackBuilderValues::regex() to support rendering re prefix strings
• CodegenEnum is no longer a CodegenClassish, as the majority of methods were not relevant
• added addVerbatim to HackBuilder. This does not auto-indent the results; one use is heredocs.