sha256 :no_check as synonym for no_checksum

[rolandwalker]

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 remove md5 checksum support #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.

[muescha]

there should be 2 different values stored in @Sums :no_check and :no_checksum for better error messages.

otherwise the error messages get a little confusing in this cases:

sha1 :no_check
sha256 2342334t34t

sha1 :no_check
sha256 :no_check