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.
