std: Remove deprecated/unstable num functionality

This commit removes all the old casting/generic traits from `std::num` that are
no longer in use by the standard library. This additionally removes the old
`strconv` module which has not seen much use in quite a long time. All generic
functionality has been supplanted with traits in the `num` crate and the
`strconv` module is supplanted with the [rust-strconv crate][rust-strconv].

[rust-strconv]: https://github.com/lifthrasiir/rust-strconv

This is a breaking change due to the removal of these deprecated crates, and the
alternative crates are listed above.

[breaking-change]

src/libsyntax/parse/mod.rs

@@ -22,8 +22,6 @@ use std::cell::{Cell, RefCell};
 use std::fs::File;
 use std::io::Read;
 use std::iter;
-#[allow(deprecated)] // Int
-use std::num::Int;
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
 use std::str;