Enhancement: add ability to patch variables defined in C code

[hab25]

Use case:

I want to customize the global value of mode-line-format while being made aware if the default value changes because that might bring improvements I can use in my customized value, or it may catch an error if the "type" of the variable changes.

[hab25]

Note that mode-line-format has a standard-value symbol property that can be looked at with custom--standard-value. Perhaps this can be used by such an enhancement (however, then the enhancement would be restricted to variables with the standard-value symbol property).

[raxod502]

It would be a bit awkward, but I suppose there is no reason we could not do this with a small amount of extra effort, and I do see why it would be useful. Thanks for the suggestion.