Sha2: Remove unecessary integer type specifications.
Sha2: Rearrage traits / impls / structs.

The code was arranged so that the core Sha2 code came first, and then
all of the various implementation of Digest followed along later. The
problem is that the Sha512 compression function code is far away from
the Sha512 Digest implementation, so, if you are trying to read over
the code, you need to scroll all around the file for no good reason. The
code was rearranged so that all of the Sha512 code is in one place and
all of the Sha256 code is in another and so that all impls for a struct
are near the definition of that struct.
Sha2: Remove the result_X() methods; move logic into the Digest impls.

The result_X() methods just calculate an output of a fixed size. They don't
really have much to do with running the actually hash algorithm until the very
last step - the output. It makes much more sense to put all this logic into
the Digest impls for each specific variation on the hash function.
Sha2: Create cryptoutil.rs and re-write the Sha2 module to make use of it.

There are 2 main pieces of functionality in cryptoutil.rs:
* A set of unsafe function for efficiently reading and writing u32 and u64
  values. All of these functions are fairly easy to audit to confirm that
  they do what they are supposed to.
* A FixedBuffer struct. This struct keeps track of input data until there
  is enough of it to execute the a function on it which expects a fixed
  block of data.

The Sha2 module was rewritten to take advantage of the new functions in
cryptoutil as well as FixedBuffer. The result is that the duplicate code
for maintaining a buffer of input data is removed from the Sha512 and
Sha256 implementation. Additionally, the FixedBuffer code is much more
efficient than the previous code was.
Sha2: Re-write the Sha2 compression functions to improve performance.

The Sha2 compression functions were re-written to execute the message
scheduling calculations in the same loop as the rest of the compression
function. The compiler is able to generate much better code. Additionally,
innermost part of the compression functions were turned into macros to
reduce code duplicate and to make the functions more concise.
Crypto: Remove DigestUtil and convert to default methods on the Digest trait.
Crypto: Add large input tests for all Digests

Create a helper function in cryptoutil.rs which feeds 1,000,000 'a's into
a Digest with varying input sizes and then checks the result. This is
essentially the same as one of Sha1's existing tests, so, that test was
re-implemented using this method. New tests were added using this method for
Sha512 and Sha256.
Crypto: Add overflow checking addition functions.

Added functions to cryptoutil.rs that perform an addition after shifting
the 2nd parameter by a specified constant. These function fail!() if integer
overflow will result. Updated the Sha2 implementation to use these functions.
Sha1: Update Sha1 to use new functionality in cryptoutil.rs
auto merge of #8174 : DaGenix/rust/digest-improvements, r=brson,graydon

Same content as #8097, but bors had an issue with that pull request. Opening a new one.