Use Fable to generate JS and TypeScript Declarations

[panesofglass]

The Gluon compiler and serializer supports a very limited number of F# and .NET types (see #32 for an example). Fable provides support for quite a lot of F# and .NET but currently does not generate TypeScript source or declaration files (see fable-compiler/Fable#805).

This issue proposes transforming Gluon into a client generator using Fable for JS compilation and (de)serialization. The idea is similar to that presented in Statically Typed Client-Server Communication with F#: Proof of Concept but using standard F# modules and attributed let-bound functions as the definitions. We could investigate use of interfaces or other options as a future enhancement.

This issue requires adding TypeScript support to Fable. As noted in the issue linked above, TypeScript 2.3 provides a TypeScript emitter from its AST. We could add support to Fable to generate TypeScript from the Fable AST as an alternative to Babel. Babel is also considering support for generating .d.ts files. Both avenues should be explored.