Clarify that the SipHash implementation has not be validated for cryptographic purposes.
Update workcache to no longer use Sha1.
Remove MD5 and Sha2. Move Sha1 and support code into librustpkg.
Add a comment to Sha1 that its not suitable for cryptogrpahic uses.
Remove dead code from cryptoutil.rs and remove macro_rules feature flag.
Remove MD4.
Integrate the code in the digest and cryptoutil modules directly into the sha1 module.
auto merge of #9744 : DaGenix/rust/remove-crypto, r=alexcrichton

Remove the Sha1, Sha2, MD5, and MD4 algorithms. SipHash is also cryptographically secure hash function and IsaacRng is a cryptographically secure RNG - I left those alone but removed comments that implied they were suitable for cryptographic use. I thought that MD4 was used for something by the compiler, but everything still seems to work with it removed, so, I guess not.

One thing that I'm not sure about - workcache.rs and workcache_support.rs (in librustpkg) both depend on Sha1. Without Sha1, the only hash function left is SipHash, so I switched that code over to use SipHash. The output size of SipHash is only 64-bits, however - much less than 160 for Sha1. I'm not sure this is a problem. Without other cryptographic hashes in the tree, I'm not sure what else to do. I considered moved Sha1 into librustpkg, but I don't know if that makes sense.

If merged, this closes #9300.