type safe option value getters

[dcbaker]

This adds a couple of methods to the OptionStore to work with options value in a type safe manner. This works by putting the type in the getter itself, this avoids the need for the pattern of:

opt = optstore.get_value_for('b_foo')
assert isinstance(opt, str), 'for mypy'

While also giving better error messages. Additionally, I've added a default= parameter, to allow not erroring when an options isn't available (a per-platform or per-compiler option). The end result is the following:

• get_value_for a type safe getter for a system value
• get_value_for_untyped a type unsafe getter for system values, works like get_value_for in current main
• get_value_for_target a type safe getter for an option that could be overwritten by a target
• get_value_for_target_untyped a type unsafe getter for an option that could be overwritten by a target, same as get_value_for_target in current main branch
• get_value_for_maybe_target a type safe getter that allows a target to be optional. This is a bit ugly, but I don't see a better way to handle this without duplicating code in a way that would lead to bugs.

this allows the following patterns to be eliminated:

opt = optstore.get_value_for_untyped(OptionKey('b_foo'))
assert isinstance(opt, str), 'for mypy'

with

opt = opstore.get_value_for(OptionKey('b_foo'), str)

(and the same for targets)

and

key = OptionKey('b_foo')
if target is not None:
    opt = opstore.get_value_for_target(target, key, str)
else:
    opt = opstore.get_value_for(key, str)

with

opt = optstore.get_value_for_maybe_target(target, OptionKey('b_foo'), str)

and

try:
    opt = opstore.get_value_for(OptionKey('b_foo'), bool)
except KeyError:
    opt = False

with

opt = optstore.get_value_for(OptionKey('b_foo'), bool, default=False)

This has been set to draft as I'm sure there will be some issues with Windows and the VS backend that need to be resolved.