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/parse/token.rs

@@ -381,6 +381,10 @@ pub enum Nonterminal {
     NtMeta(P<ast::MetaItem>),
     NtPath(Box<ast::Path>),
     NtTT(P<ast::TokenTree>), // needs P'ed to break a circularity
+    // These is not exposed to macros, but is used by quasiquote.
+    NtArm(ast::Arm),
+    NtImplItem(P<ast::ImplItem>),
+    NtTraitItem(P<ast::TraitItem>),
 }
 
 impl fmt::Debug for Nonterminal {
@@ -396,6 +400,9 @@ impl fmt::Debug for Nonterminal {
             NtMeta(..) => f.pad("NtMeta(..)"),
             NtPath(..) => f.pad("NtPath(..)"),
             NtTT(..) => f.pad("NtTT(..)"),
+            NtArm(..) => f.pad("NtArm(..)"),
+            NtImplItem(..) => f.pad("NtImplItem(..)"),
+            NtTraitItem(..) => f.pad("NtTraitItem(..)"),
         }
     }
 }