remove ltxcmds dependency

[davidcarlisle]

This PR removes the dependency on the ltxcmds pakage.

Mostly ltxcmds provides LaTeX commands for other formats so is not needed in hyperref. (Just use \@firstoftwo rather than \ltx@firstoftwo etc)

It does provide a few extra features, here taken from expl3, and given Hy@ names

\let\Hy@LocalAppendToMacro\tl_put_right:Nn
\let\Hy@GlobalAppendToMacro\tl_gput_right:Nn
\let\Hy@ifempty\tl_if_empty:nTF

Also a couple of afterfi commands defined directly in the Hy@ prefix

\long\def\Hy@ReturnAfterFi#1\fi{\fi#1}
\long\def\Hy@ReturnAfterElseFi#1\else#2\fi{\fi#1}

The main advantage is an iterative step towards aligning the hyperref coding with the main kernel sources.

[u-fischer]

Looks good. The generic hyperref driver in the pdfresources contains some ltxcmds too (I removed them now), so that should be updated first.