Introduce rustc_lexer

The idea here is to make a reusable library out of the existing
rust-lexer, by separating out pure lexing and rustc-specific concerns,
like spans, error reporting an interning.

So, rustc_lexer operates directly on `&str`, produces simple tokens
which are a pair of type-tag and a bit of original text, and does not
report errors, instead storing them as flags on the token.

src/test/ui/parser/lex-bad-numeric-literals.stderr

@@ -53,10 +53,10 @@ LL |     0o;
    |     ^^
 
 error: expected at least one digit in exponent
-  --> $DIR/lex-bad-numeric-literals.rs:12:8
+  --> $DIR/lex-bad-numeric-literals.rs:12:5
    |
 LL |     1e+;
-   |        ^
+   |     ^^^
 
 error: hexadecimal float literal is not supported
   --> $DIR/lex-bad-numeric-literals.rs:13:5

src/test/ui/parser/raw-byte-string-eof.stderr

@@ -1,8 +1,8 @@
 error: unterminated raw string
-  --> $DIR/raw-byte-string-eof.rs:2:6
+  --> $DIR/raw-byte-string-eof.rs:2:5
    |
 LL |     br##"a"#;
-   |      ^ unterminated raw string
+   |     ^ unterminated raw string
    |
    = note: this raw string should be terminated with `"##`
 

src/test/ui/parser/raw-byte-string-literals.stderr

@@ -11,10 +11,10 @@ LL |     br"é";
 LL |     br"é";
    |        ^
 
-error: found invalid character; only `#` is allowed in raw string delimitation: ~
+error: found invalid character; only `#` is allowed in raw string delimitation: ~
   --> $DIR/raw-byte-string-literals.rs:6:5
    |
-LL |     br##~"a"~##;
+LL |     br##~"a"~##;
    |     ^^^^^
 
 error: aborting due to 3 previous errors

src/test/ui/parser/raw-str-delim.stderr

@@ -2,7 +2,7 @@ error: found invalid character; only `#` is allowed in raw string delimitation:
   --> $DIR/raw-str-delim.rs:2:5
    |
 LL |     r#~"#"~#
-   |     ^^
+   |     ^^^
 
 error: aborting due to previous error