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/mod.rs

@@ -28,8 +28,6 @@ use std::path::{Path, PathBuf};
 use std::rc::Rc;
 use std::str;
 
-use rustc_i128::u128;
-
 pub type PResult<'a, T> = Result<T, DiagnosticBuilder<'a>>;
 
 #[macro_use]