auto merge of #10143 : chris-morgan/rust/filename-consistency, r=huonw

New standards have arisen in recent months, mostly for the use of
rustpkg, but the main Rust codebase has not been altered to match these
new specifications. This changeset rectifies most of these issues.

- Renamed the crate source files `src/libX/X.rs` to `lib.rs`, for
  consistency with current styles; this affects extra, rustc, rustdoc,
  rustpkg, std, syntax.

- Renamed `X/X.rs` to `X/mod.rs,` as is now recommended style, for
  `std::num` and `std::terminfo`.

- Shifted `src/libstd/str/ascii.rs` out of the otherwise unused `str`
  directory, to be consistent with its import path of `std::ascii`;
  libstd is flat at present so it's more appropriate thus.

While this removes some `#[path = "..."]` directives, it does not remove
all of them, and leaves certain other inconsistencies, such as `std::u8`
et al. which are actually stored in `src/libstd/num/` (one subdirectory
down). No quorum has been reached on this issue, so I felt it best to
leave them all alone at present. #9208 deals with the possibility of
making libstd more hierarchical (such as changing the crate to match the
current filesystem structure, which would make the module path
`std::num::u8`).

There is one thing remaining in which this repository is not
rustpkg-compliant: rustpkg would have `src/std/` et al. rather than
`src/libstd/` et al. I have not endeavoured to change that at this point
as it would guarantee prompt bitrot and confusion. A change of that
magnitude needs to be discussed first.