Vendor rust-crypto aes/soft implementation

[briansmith]

The code is at https://github.com/RustCrypto/block-ciphers/tree/master/aes/src/soft.

• The code should be placed in src/rust_crypto/aes/soft/ in the ring sources.
• The first commit should add the two source files, unmodified. They will not successfully build. Include the full commit hash from the upstream repository in the commit message.
• Remove hazmat and other code that will never be needed. Retain the decryption code; even though it will be dead code; the future AES-CBC code will use it.
• Add src/rust_crypto/aes/soft/block.rs to define the Block type in a way that does not use the cipher crate.
• Replace types FixsliceBlocks and BatchBlocks with types that do not use the cipher crate.
• Adapt ring::aead::aes to use the newly-vendored code instead of aes_nohw.c. Remove aes_nohw.c, aes/internal.h, and all references to the files from build.rs and Cargo.toml.

[briansmith]

The change that implements this should be done in a single PR. I will use diff against the latest commit of the aes crate sources on GitHub to verify that the first commit is exactly identical to what's in the upstream source. Then I will review each commit in the PR as though it were its own PR. So basically the submitter needs to be prepared to do a lot of rebasing through the review process if there are many rounds of review.