cexpr's symbol tables currently aren't readily extended, and dynamic
scoping is usually a good thing anyway. The chain of contexts is walked
when a symbol is not found in the current context's symtab, but minor
efforts are made to avoid checking the same symtab twice (usually cased
by cloning a context but not updating the symtab).
This allows plist objects to be accessed directly from cexpr expressions
using struct.field syntax for dictionary objects and array[index] syntax
for array objects.
The way I wound up using the field meant that exprctx should not "own"
the hashlinks chain, but rather just point to it. This fixes the nasty
access errors I had.
It is capable of parsing single expressions with fairly simple
operations. It current supports ints, enums, cvars and (external) data
structs. It is also thread-safe (in theory, needs proper testing) and
the memory it uses can be mass-freed.
