feat(ts-bindings)!: now powered by ContractClient

Most logic in the TS Bindings has now been implemented in stellar-sdk as
a `ContractClient` that dynamically fetches the contract's XDR and
generates JS for each contract method at runtime. This means logic like
`AssembledTransaction` now lives entirely in stellar-sdk, and can be
relied on by each library generated by TS Bindings, rather than
duplicating this logic in each of these libraries. TS Bindings now has a
much smaller focus—basically, only to generate the types.

This shrinks the TS Bindings, importing and using this new
ContractClient logic.