AMTL updates and changes to adapt for them to fix menu crashes (bug 5921, r=psychonic).

AMTL:
Removes isVoid from AString.
Fixes support inserting to Vector at length.

--HG--
extra : rebase_source : 02805fad60c2b759a0e2e91c081144854a8e0b54
Fix uninit'd memory issues in AMTL AString and Vector (bug 5921, r=psychonic).

--HG--
extra : rebase_source : 17a57dcf815cb3798eaa13af5f4af44fb1930f86