Preliminary support for matrix multiplication.

[benninkrs]

Preliminary support for matrix multiplication.

• New methods for * for combinations of OffsetArray and AbstractArray Unfortunately this introduces method ambiguities with Base.
• As an alternative, tried defining a new promote_rule, but somehow it doesn't get invoked.

New convenience constructors

• Wrap any (non-offset) array as an OffsetArray with standard indices
• Remove layer of indirection when constructing with explicit offsets

Cosmetic edits to constructor section for improved readability

[benninkrs]

@timholy As you can see from commit message I tried a couple things, neither of which worked as well as I'd hoped. If you have advice either on how to resolve the method ambiguities, or on how to make a promote_rule get invoked, I can try to improve it.