Problem: some contracts contain functionality present in our
AbstractLedger and ManagedLedger contracts. If we want to reuse
that functionality we have to depend on whole `lorentz-contracts`
package which contains some stuff besides these ledgers. So current
structure of packages adds unnecessary dependencies to contracts
that reuse ledger code.

Solution:
1. Create `morley-ledgers` package. I decided to avoid naming
Lorentz there because one day it may be implemented with Indigo.
Probably it will always be based on `morley`, hence it is in the name.
It contains ManagedLedger and AbstractLedger.
2. Move `contractConsumer` into `Lorentz.Test`. It is pretty small and
is generally useful for testing.
3. Also rename `lorentz-contracts-test` to `morley-ledgers-test`
because it contains tests only for `morley-ledgers`.

Note that I mostly preserve module names even if they refer to
Lorentz. Currently contracts are still implemented using Lorentz and
we don't want to make unnecessary breaking changes.

Also note that I didn't bother changing anything in pkgs.nix and
didn't test `nix` side of things because primary user of it is George
and according to him he will nixify the repo differently.