[#78] Add `approveCAS` to ManagedLedger

Description

ManagedLedger contract has approve entrypoint as specified in FA1.2. It sort of prevents front running attack by prohibiting changing allowance from non-zero to non-zero. However, that's not a perfect solution for a couple of reasons:

1. Sometimes it is convenient to change from non-zero to non-zero.
2. If two transactions change allowance from n to 0 and then from 0 to m, they will be accepted, but it can be dangerous if they get into the same block or adjacent blocks.

Sometimes one may want to have a safer version of approve which requires passing the expected current allowance value. Since our managed ledger is a library for writing managed-ledger-alike contracts, it makes sense to add approveCAS there to let others add it to their contracts if they want.

Here I implement approveCAS and add it to ManagedLedger. There seems to be no sense to extract some part of approve into a new function. In approveCAS we only need setAllowance. doc usage should be different, AllowanceParams is constructed from another data type, non-zero to non-zero check is not needed.

Related issue(s)

Resolves #78 (closed)

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