`sha256 :no_check` as synonym for `no_checksum`

Introduces new form `sha256 :no_check` as a synonym for `no_checksum`.
Backend code also cleaned up to use self-documenting symbol instead of special-behavior zero.

The new syntax is intentionally left undocumented.  The plan would be to have
this code in release for at least a month before transitioning Casks to use it.

Rationale: simplification.
 - DSL declaration `md5` was removed in #2931.
 - `sha1` is deprecated, we may as well move towards its removal.
 - With this PR, the oddball `no_checksum` declaration can also be removed.
 - That's 3 fewer declarations in the DSL, with equivalent functionality.