@select macro fixes

[durcan]

General:
Syntax change from if/else to =>

Blocking:

1. moved evaluation of body expression to original parent task.

Non Blocking:

1. Throw an error if more than one default case
2. Always check default case last regardless of written position
3. Allow resulting expression to be anything (previously it had to
   evaluate to a valid rvalue)

[durcan]

These are the select macro fixes discussed in issue #13763.

[brenhinkeller]

It looks like @select has been completely removed sometime in the intervening years, so I'm going to go ahead and assume it's safe to close this (though feel free to reopen if I'm mistaken!)

[vtjnash]

@select was never merged (this was a PR targeting a different PR)