Add quasi quotation about language-rust

[harpocrates]

Here is a simple example of why we need the ViewPatterns approach:

> let x = [expr| 1 |]
> let y = [stmt| $x; |]
<interactive>:42:9: error:
    • Exception when trying to run compile-time code:
        Subst x
      Code: Language.Haskell.TH.Quote.quoteExp stmt " $x; "
    • In the quasi-quotation: [stmt| $x; |]

Even more insidiously (check -ddump-splices and you'll see the error is "leaking")

> let x = [expr| 1 + 2 |]
> let y = [expr| $x * 3 |]
<interactive>:11:15: error:
    • Couldn't match expected type ‘Language.Rust.Data.Position.Span’
                  with actual type ‘Language.Rust.Syntax.AST.Expr
                                      Language.Rust.Data.Position.Span’
    • In the fifth argument of ‘Language.Rust.Syntax.AST.Binary’, namely
        ‘x’
      In the expression:
        (Language.Rust.Syntax.AST.Binary
           []
           Language.Rust.Syntax.AST.MulOp
           x
           (Language.Rust.Syntax.AST.Lit
              []
              (Language.Rust.Syntax.AST.Int
                 Language.Rust.Syntax.AST.Dec
                 3
                 Language.Rust.Syntax.AST.Unsuffixed
                 (Language.Rust.Data.Position.Span
                    (Language.Rust.Data.Position.Position 6 11 21)
                    (Language.Rust.Data.Position.Position 7 11 22)))
              (Language.Rust.Data.Position.Span
                 (Language.Rust.Data.Position.Position 6 11 21)
                 (Language.Rust.Data.Position.Position 8 11 23)))
           x)
      In an equation for ‘y’:
          y = (Language.Rust.Syntax.AST.Binary
                 []
                 Language.Rust.Syntax.AST.MulOp
                 x
                 (Language.Rust.Syntax.AST.Lit
                    []
                    (Language.Rust.Syntax.AST.Int
                       Language.Rust.Syntax.AST.Dec
                       3
                       Language.Rust.Syntax.AST.Unsuffixed
                       (Language.Rust.Data.Position.Span
                          (Language.Rust.Data.Position.Position 6 11 21)
                          (Language.Rust.Data.Position.Position 7 11 22)))
                    (Language.Rust.Data.Position.Span
                       (Language.Rust.Data.Position.Position 6 11 21)
                       (Language.Rust.Data.Position.Position 8 11 23)))
                 x)

[harpocrates]

Alright. I've mulled over this for a week and I've come up with a new approach, which isn't a hack:

• wrap all recursive ASTs in m (so data ExprT m a = PlusT (m (ExprT m a)) (m (ExprT m a)) a...)
• update production rules to be parametrized over Context m => ExprT m Span where Context defines functions that need to look at the contents of parsed values (like the path case of patterns which checks for one identifier patterns)
• make type synonyms (type Expr = ExprT Identity) and patterns (pattern Plus x y = PlusT (Identity x) (Identity y)). This also means fixing how Haddock handles pattern synonyms
• re-make Quote!

This may take some time...

[harpocrates]

This won't make the 0.1.0.0 release. Instead, Language.Rust.Quote quasiquoters will not support $x or $x:ty splices/extractors.