More work on decoupling the general runtime from Interpreter. The goal
is becoming clearer. Interpreter should be one possible way to execute
code inside a VM. In the future we might have other ways :^)
This patch moves the exception state, call stack and scope stack from
Interpreter to VM. I'm doing this to help myself discover what the
split between Interpreter and VM should be, by shuffling things around
and seeing what falls where.
With these changes, we no longer have a persistent lexical environment
for the current global object on the Interpreter's call stack. Instead,
we push/pop that environment on Interpreter::run() enter/exit.
Since it should only be used to find the global "this", and not for
variable storage (that goes directly into the global object instead!),
I had to insert some short-circuiting when walking the environment
parent chain during variable lookup.
Note that this is a "stepping stone" commit, not a final design.
Currently only supports setting the foregound and the background colours.
This patch also unifies `foreground_color' and `background_color' used
throughout to a `Format' struct, in hopes of getting more formatting
options one day :P
This commit adds a generic interface for cell types and hooks it up.
There is no way to set these from the UI, and so they're not saved
anywhere yet.
Also implicitly converts numeric values (strictly integers) to numeric
javascript values, as numbery-looking + numbery-looking === string is
not very interesting. :^)
