Instead of having hand-written DynamicScope subtypes that use
fields instead of an array, this logic generates those subclasses
as needed.

Currently it only overrides the base getValue and setValue methods
and DynamicScope now implements the other previously-abstract
methods by calling those two. This is not optimal for generic
access via those indexed methods, but the expectation is that JIT
can just go after the fields directly. The additional overrides
would likely be useful for the interpreter, though.

See #4167.

* Get and set of depth zero, offset = 0..9
* Flip override of sets to void versions
* Add test for the key methods

* Limit specialized subclasses to 50 wide.
* Move to a generator class.
* Cache construction logic to StaticScope.
* Cache constructor in StaticScope.
* Reduce duplication in code gen.
* Move offset error to a shared method in generated classes.
* Use a single OneShotClassLoader since we never dereference.
* Misc perf, style tweaks.

…opes