Handle global variables.

Global variables will be stored in an array which is kept in register 0.
The linker will later translate each library-ref into an integer, which
is used as an index into the globals table.

This design makes implementing eval quite straightforward, the eval
bytecode will accept a globals table and a bytevector of bytecode, save
the current set of registers, set register 0 to the new globals table,
and begin executing the given bytecode. When eval is finished, it will
restore the saved registers and return to the previous instruction
pointer. In this way, calling eval can create a call stack.

We could think about implementing function calls with eval, and it's
cool that it would work, but the CPS transformation mostly makes this
irrelevant. It's interesting to think about implementing an interpreter,
where a function call would simply eval the function's bytecode, and the
function call stack would be handled automatically.

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