Add functions exposing constants for all operation/primitive pairs

Wrappers for other languages can't access constants defined by C
preprocessor macros, so they must be exposed as functions. This was
already done for the "default" implementation of each operation, but
this commit adds functions for (almost) all of the underlying
primitives.

A few are currently excluded due to problems with the automated script
used to generated this commit. They are:

  * crypto_onetimeauth_poly1305
  * crypto_generichash_blake2b
  * crypto_verify16
  * crypto_verify32
Fix definition of crypto_box_curve25519xsalsa20poly1305_macbytes

This function definition was created by an automated script that
incorrectly handled a corner case.
Add remaining functions for looking up constants

The automated script that generated functions for looking up #define'd
constants didn't handle edge cases in these files, so these have been
added by hand. They're thus either more likely or less likely to
contain mistakes (depending on one's particular point of view).
Merge remote-tracking branch 'stouset/add-lookup-methods-for-all-constants'

* stouset/add-lookup-methods-for-all-constants:
  Add remaining functions for looking up constants
  Fix definition of crypto_box_curve25519xsalsa20poly1305_macbytes
  Add functions exposing constants for all operation/primitive pairs