Refactor the C API for invoking Wren methods from C code. Remove the old
wrenGetMethod and wrenCallVarArgs functions that used a type-string-based
argument marshalling system. Introduce a new slot-based approach where
callers first call wrenEnsureSlots to reserve space, then use wrenSetSlot___
functions to place the receiver and arguments on the stack, and finally call
wrenCall with a handle created by wrenMakeCallHandle. Results are retrieved
via wrenGetSlot___. This change simplifies the VM internals, eliminates
varargs parsing overhead, and yields a 185% speed improvement in the call
benchmark. Update all internal modules (scheduler, io, timer) and test files
to use the new API. Also fix fiber suspension to clear the API stack pointer
and adjust the interpreter to store the final fiber result at stack[0] for
C API access.
Bob Nystrom
9f48ac681c