[ add ] functorial change-of-base from `S-YModule` to `R-YModule` induced by an `XRing` homomorphism from `R` to `S`

[jamesmckinna]

See discussion on #2817 concerning the 'diagonal' map.

The action, given appropriate h : XHom (R, S) takes (all the left-right variants on) scalar multiplication s * m in S-YMod to λ r m → h r * m in R-YMod.

NB:

• X varies over the Ring, Semiring, NearSemiring... etc. Bundles defining 'scalar' structure for
• Y varying over Module, (Pre){Left|Right}Semimodule etc.

Issues:

• where should this live?
• naming?
• parametrisation wrt Raw bundles vs. fully-articulated Bundles, as usual for homomorphism-related constructions
• etc.