set-variable could use some of defcustom's typing to provide completion

[Drew Adams]

I don't propose we try to take this too far, but in the case of some simple
defcustom :type specs, `set-variable' could use the type info to provide
completion.

For instance, if a `choice' of `const' values is used, the string
representations of those values could be made available as completions for
input to `set-variable'.

E.g. (choice (const :tag "Yes" 'some-long-option-meaning-yes)
             (const :tag "No"  'some-long-option-meaning-ask)
             (const :tag "Ask" 'some-long-option-meaning-ask))

You would be able to complete your input to the only valid inputs.

Completion could be lax or strict, depending on the type. In the case of a
choice of (only) `const's, it could be strict. For some other types, it
would be completely lax, letting you input anything you like.

Yes, I realize that the primary recommended UI for setting options is
Customize, but `set-variable' exists and is quite handy. With completion, it
would be even handier.