Typestate preconditions in ```std::sha1```

[catamorphism]

std::sha1 has a number of FIXMEs about functions that should have typestate preconditions. Once we decide what our stance is on typestate in the standard libraries, either add them or delete the FIXMEs.

[nikomatsakis]

Not relevant as of commit 41a21f0, which removed current incarnation of typestate.

[nikomatsakis]

well, still relevant until the FIXMEs are removed.