syntax: don't parse numeric literals in the lexer

This removes a bunch of token types. Tokens now store the original, unaltered
numeric literal (that is still checked for correctness), which is parsed into
an actual number later, as needed, when creating the AST.

This can change how syntax extensions work, but otherwise poses no visible
changes.

[breaking-change]

src/libsyntax/codemap.rs

@@ -96,7 +96,7 @@ pub struct Span {
 
 pub static DUMMY_SP: Span = Span { lo: BytePos(0), hi: BytePos(0), expn_info: None };
 
-#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash)]
+#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub struct Spanned<T> {
     pub node: T,
     pub span: Span,