A new Extension enum value for codegen

[aseemr]

This PR adds a new enum value Extension to the --codegen option. When F* is invoked with --codegen Extension, it will dump a tuple (bs, decls) in the binary format to a .ast file. Here bs is the bindings in the extraction environment and decls are the ML AST declarations.

The PR also changes the extension extractor interface:

• The extraction extractor now takes in sigelt and returns ML decls.
• Extension extractor provides two functions: extract_sigelt and extract_sigelt_iface.

Both these changes are for working toward extraction of Pulse.