add SolSavingEnvironment, halving pickle size

[bqpd]

Per #1497

@pgkirsch this halves pickle size without removing any solution information. Between this and the compression I'd expect that 110MB file to drop to less than 10MB.

[bqpd]

test models please

[pgkirsch]

@bqpd just curious: what do the "construction/solve attributes" allow you to do with an already-computed solution? Re-solve without needing the original model object?

[bqpd]

The following constraint attributes are removed: ["mfm", "pmap", "bounded", "meq_bounded", "v_ss", "unsubbed", "varkeys"].

mfm: maps how monomials simplified during substitution so that sensitivities can be properly mapped back
pmap: maps how the constant term (if any) was taken to the <= 1 side and divided out
bounded: which variables were upper or lower bounded by this constraint at its creation
meq_bounded: which variables were conditionally upper or lower bounded through a monomial equality
v_ss: the individual variable sensitivities in each constraint
unsubbed: presubstitution polynomial representations of the constraint
varkeys: set of varkeys in the constraint

All of these except for v_ss are recreatable by initing a new constraint with the left, oper, and right of the saved version.

[pgkirsch]

Ah awesome, thanks for the detailed explanation!