pp: print spaces around '=' in 'extern mod blah = "blah"'
lex raw string literals, like r#"blah"#

Raw string literals are lexed into regular string literals. This is okay
for them to "work" and be usable/testable, but the pretty-printer does
not know about them yet and will just emit regular string literals.
document raw string literals in tutorial.md and rust.md
add vim syntax highlighting support for raw string literals
add token::LIT_STR_RAW(ident, num of # symbols)

Treat it as a synonym for LIT_STR for now.
add new enum ast::StrStyle as field to ast::lit_str

For the benefit of the pretty printer we want to keep track of how
string literals in the ast were originally represented in the source
code.

This commit changes parser functions so they don't extract strings from
the token stream without at least also returning what style of string
literal it was. This is stored in the resulting ast node for string
literals, obviously, for the package id in `extern mod = r"package id"`
view items, for the inline asm in `asm!()` invocations.

For `asm!()`'s other arguments or for `extern "Rust" fn()` items, I just
forbid raw string literals for now, mostly because it seemed
disproportionally cumbersome to thread that information through the
string processing that happens with those string literals, given the
limited advantage raw string literals would provide in these positions.

The other syntax extensions don't seem to store passed string literals
in the ast, so they discard the style of strings they parse for now.
allow raw string literals in `extern r"Rust"` positions
pp: add tests for raw strs in non-expression positions