Strings should not be allocated to a table

[pufferfish101007]

Rather than growing the strings table every time we need to allocate a string off of the stack, strings should be stored in globals/locals that are unique to each variable (following #31, there should be no other time at which strings need to be allocated). This might increase copying of strings but that should have effectively zero overhead because externrefs are immutable references.
This should also speed up string access from boxed variables, because we will no longer need to store a pointer to the string index, as the global/local index of a string can be known at compile time.

[pufferfish101007]

It should be noted that, with the current implementation of lists, the goal of completely removing the strings table is not feasible in the form outlined above. Possible ways of going about this:

• just keep the strings table, but only use it for allocation to lists of boxed types (simple, more allocation = bad (+ memory leaks))
• allocate another array (or table) for each list of boxed types, of element type ref null extern, and when an element of the list is marked as being a string, get it from the corresponding index (no more memory leaks, unsure if more performant than monolithic table or not)
• change lists of boxed types to store a nullable struct of some sort, containing fields for a tag, f64 value, i32 value and nullable externref (no more silly string management needed, but boxed access is slower, also takes up more memory, doesn't work with non-GC-array lists)
• just keep the strings table, but ref count/garbage collect manually (no thank you)

[pufferfish101007]

This also isn't easy to do when considering variables returned from warped procedures that use control_stop, as we might assign to one variable but then need the return variables to be boxed. Since we don't know the callsite ahead of time, we can't allocate a single global to any return variable ahead of time; one possible option is to pass in a mutable nullable externref as a param, and modify this if we want to return a boxed string; I'm not convinced that this is much better.