lang: codegen: redesign variable definition/resolution system to support capturing

the codegen scope system has been removed. instead, each generator state in the stack,
from the current state backwards, is informed when a variable is defined, resolved, or
captured.

when a variable is defined, the state stack is traversed back-to-front (current generator
first). each state has a chance to record the variable definition. once one state has
signalled that it has recorded the variable definition, the traversal ends.

when a variable is resolved, the state stack is traversed back-to-front (current generator
first). each state is asked whether or not it recognises the variable identifier being resolved.
if a state has the variable in question defined, it returns information about the variable
definition, and the traversal stops.

once a variable has been resolved, the state stack is traversed front-to-back (current generator
last), starting from the generator /after/ the one that provided the variable definition. each
generator in the iteration is given the chance to adjust the variable information, or generate
IR in response to the variable being accessed. this is used to implement variable capture,
where the state of a variable in the enclosing context is captured for later use.

lang/codegen/block.c

@@ -45,8 +45,6 @@ static enum ivy_status state_init(
 	uintptr_t argv, void *argp)
 {
 	struct block_codegen_state *block = (struct block_codegen_state *)state;
-	block->s_scope = codegen_push_scope(gen);
-
 	return IVY_OK;
 }
 
@@ -54,8 +52,6 @@ static enum ivy_status state_fini(
 	struct ivy_codegen *gen, struct code_generator_state *state,
 	struct mie_value **result)
 {
-	codegen_pop_scope(gen);
-
 	return IVY_OK;
 }