Support for custom syntax extensions

[nikswamy]

This PR adds support for custom syntax extensions.

You can write in place of a top-level declaration

```your_dsl_name
whatever syntax
```

In a compiler plugin, you can call

val register_extension_parser (extension_name:string) (parser:extension_parser) : unit

passing "your_dsl_name" to register a custom parser for whatever syntax your DSL chooses.

The extension_parser has type:

type extension_parser = 
   open_namespaces_and_abbreviations ->
   contents:string ->
   p:FStar.Compiler.Range.range ->
   either error_message decl'

receiving:

1. open_namespaces_and_abbreviations: The open namespaces and module abbreviations in the current scope environment

2. the concrete syntax of the your DSL as a string

3. The source range at which the string begins (so you can report syntax errors relative to the position in the source file)

and returning

either an error message or a FStar.Parser.AST.decl'. If the latter, then this decl' is processed by the rest of the compiler pipeline as is.

For an example of this mechanism see: https://github.com/FStarLang/steel/blob/nik_ranges/share/steel/examples/pulse/CustomSyntax.fst