Interpolate AST nodes in quasiquote.

This changes the `ToTokens` implementations for expressions, statements,
etc. with almost-trivial ones that produce `Interpolated(*Nt(...))`
pseudo-tokens. In this way, quasiquote now works the same way as macros
do: already-parsed AST fragments are used as-is, not reparsed.

The `ToSource` trait is removed. Quasiquote no longer involves
pretty-printing at all, which removes the need for the
`encode_with_hygiene` hack. All associated machinery is removed.

A new `Nonterminal` is added, NtArm, which the parser now interpolates.
This is just for quasiquote, not macros (although it could be in the
future).

`ToTokens` is no longer implemented for `Arg` (although this could be
added again) and `Generics` (which I don't think makes sense).

This breaks any compiler extensions that relied on the ability of
`ToTokens` to turn AST fragments back into inspectable token trees. For
this reason, this closes #16987.

As such, this is a [breaking-change].

Fixes #16472.
Fixes #15962.
Fixes #17397.
Fixes #16617.

src/libsyntax/ast.rs

@@ -89,12 +89,6 @@ impl Ident {
     pub fn as_str<'a>(&'a self) -> &'a str {
         self.name.as_str()
     }
-
-    pub fn encode_with_hygiene(&self) -> String {
-        format!("\x00name_{},ctxt_{}\x00",
-                self.name.usize(),
-                self.ctxt)
-    }
 }
 
 impl fmt::Debug for Ident {