Avoid some invalidations related to Base.hash

[lgoettgens]

According to the julia docs, new types should implement the 2-argument version of hash, as this allows for composeability of hashing and there exists a generic delegation from the 1-arg to the 2-arg version in julia base. Not adhering to this leads to a bunch of invalidations, i.e. basically all methods that uses Base.hash in their body need to be re-compiled once this method from AlgebraicSolving is loaded (cf. oscar-system/Oscar.jl#2525).

Furthermore, the julia docs state that Base.hash should always return a UInt (i.e. a 64-bit unsigned int on most systems). I added compliance with that by splitting the function into one named Base.hash that returns a UInt, and one named hash_monomial that returns a MonHash (aka UInt32).

cc @fingolfin

[ederc]

Thanks!