Don't export Stdlib

[kris-brown]

We've been reexporting Stdlib, which is great for things like easily using ThCategory in Catlab. However, there are a lot of things in Stdlib and they are likely to be more 'toy implementation' than the analogous thing we want in Catlab. For example, FinSetC is a great name for the category of finite sets and functions, but in Catlab it would be better to define this with the FinSet machinery that is defined there (the version in GATlab.Stdlib is a toy version of SkelFinSet). In general, there have been many name collisions (e.g. op) that seem harder to deal with than occasional explicit using GATlab.Stdlib: ThCategory, so I think it would be best to not reexport Stdlib.

For reference, here are all the names included in Stdlib:

BoolRing
DerivedModels
FinMatC
FinSetC
GATC
IntMonoid
IntNat
IntNatPlus
IntPreorder
IntPreorderCat
Maps
NatPlusMonoid
NothingC
OpC
OpCat
OpFinSetC
PreorderCat
SliceC
SliceOb
StdDerivedModels
StdModels
StdTheories
StdTheoryMaps
Stdlib
SwapMonoid
ThAb
ThAscCat
ThBooleanRing
ThCMonoid
ThCRig
ThCategory
ThClass
ThCommRModule
ThCommRing
ThDiffRing
ThDivisionRing
ThElementary
ThEmpty
ThGraph
ThGroup
ThLawlessCat
ThLawlessMonCat
ThLeftModule
ThMagma
ThModule
ThMonoid
ThNat
ThNatPlus
ThNatPlusTimes
ThPreorder
ThRightModule
ThRing
ThSemiGroup
ThSemiRing
ThSet
ThStrictMonCat
ThThinCategory
ZRing
op