Implement expanded-fix for keyval

[Skillmon]

This PR implements the \unexpanded\expanded fix for \keyval_parse:nnn to make it alignment safe. As a fall back, fix number 2 from #896 is used (with proper adjustments to the error recovery for blank key names).

I ran the testsuite locally on the code used if \expanded isn't available. One should be aware that the testsuite will henceforth only test the expanded variant (at least without manual modifications).

[blefloch]

I ended up using parts of the ideas in this pull request but with modifications to keep the code more compact and avoid having twice the same code simply with extra \group_align_safe_begin:/_end:. Please take a look at commits ec1e3f9 and 05cc492 and check if they seem correct.

[Skillmon]

I think it looks good, but only took a quick glance on my mobile. This evening I can take another look and give proper feedback.

[Skillmon]

@blefloch took another look, all is looking good. Also, I think the definition of \@@_blank_key_error:w could be even more simplified, since the error-case doesn't have to be fast, imho. So the argument grabbing optimization there is really unnecessary. We could go with a single definition, namely:

\cs_new:Npn \@@_blank_key_error:w #1 \@@_loop_other:nnw
  {
    \__kernel_msg_expandable_error:nn { keyval } { blank-key-name }
    \@@_loop_other:nnw
  }