Pass invoke context to GetElement

This allows us to implement complex structure objects that require some
context (e.g., Anacreon objects as datums).
Added crash detection to Hexe primitive function calls
Fixed bug with re-entrant primitives

After calling a primitive that called HexeLisp, the code bank field was
not preserved properly.
Added sort and other functions to HexeLisp
Fixed a crash bug in Esper processing thread