Bump version, upgrade bootstrap

This commit updates the version number to 1.17.0 as we're not on that version of
the nightly compiler, and at the same time this updates src/stage0.txt to
bootstrap from freshly minted beta compiler and beta Cargo.

src/libsyntax/parse/parser.rs

@@ -65,8 +65,6 @@ use std::path::{Path, PathBuf};
 use std::rc::Rc;
 use std::slice;
 
-use rustc_i128::u128;
-
 bitflags! {
     flags Restrictions: u8 {
         const RESTRICTION_STMT_EXPR         = 1 << 0,