new types should go at end of struct

This is an initial proposal for supporting typed globals based on #43455.
The syntax for this is either `global x::T` or just doing `x::T = 1` in
global scope. It is even supported to add these annotations to globals
from inside functions. The type declaration will then be applied when
the method is defined and inside the function body conversion to the
specified type will happen automatically, similar to type annotations
for local variables.

This conversion will not be applied if the assignment is not done inside
the same scope of the type annotation however. This could potentially be
supported as well, but the problem is that this would mean any
assignment to a global variable - typed or not - would need to go
through a call to `convert` first, since lowering can't know if a given
binding already has a type annotation. This probably wouldn't be a good
thing for latency and sysimage size and would add an invalidation risk.

It is allowed to refine a type annotation, but there will be a warning.
Widening the type or changing it to any other type that is not a subtype
of the previous one is an error, since that could cause cached compiled
code to become invalid.

replaces #43455
closes #964

Lowering split up `i = i - 1` using an additional SSA value here, which
should be completely harmless, but broke this test.

Change lowering s.t. types are always frozen before functions writing to
a global are defined. `get_binding_type` now errors if the binding is
still uninitialized. Also make multiple type declarations in local scope
a runtime error.