More code review and cleanup of host.rs module

[graydon]

Today I didn't get as much done -- had some errands and a doctor appointment -- but my goal was to get through the residual bits of host.rs and I did that at least. Along the way I removed the allows that enabled un-warned dead code and unused variables, and found some stuff:

• Two missing error checks on linear-memory "unpack" vector lengths. These don't represent a major vulnerability, but they do allow users to do an unpack to a mismatched vector length, which could hide a bug. A footgun at least. Fixed.
• A dead cost type, based on a previous iteration of the secp256k1 interface. Filed ComputeEcdsaSecp256k1Key is never used #1087 to track removing it.
• Some diagnositcs weren't using the names we so meticulously pass down to them. Just throwing them out. Fixed.
• There was a bunch of dead code and over-public interfaces. Fixed.