Improve the method of typecheck blocks

[jparoz]

A better model for typechecking blocks might be to introduce a type Closure which is similar to the current Scope (or SubScope). This contains all the locals declared, globals assigned, and globals constrained in the block. Typechecking a block returns this closure object; this can then be applied to the parent scope (closure), or interpreted similar to the current Scope.

Note that the same variable name could appear as a local, assigned and constrained global, e.g.:

do
    local _ = x .. "foo"
    x = 333
    local _ = x + 5
    local x = "sss"
end

This is valid, but the x before and after the global assignment, as well as the local x, are all distinct in view of the closure.