implement local key for \symdef

[kohlhase]

The \symdef macro has had the local key that made the semantic macro local to the current group by not exporting it from the module. This has never been used much, so it has never been implemented in LaTeXML. But now it is part of the \vardef concept (see #35), which is very important for SMGloM, so we should implement it.
I do not think that this should be very difficult; I guess we should just make a conditional if the local key is set and then refrain from registering the macro from the imports list (but I cannot quite see where this is one in the code of modules.sty.ltxml.

[kohlhase]

This issue was moved to slatex/LaTeXML-Plugin-sTeX#44