This allows specifying different "policy paths" for the internal and external
descriptors, and adds additional checks to make sure they are compatibile (i.e.
the timelocks are expressed in the same unit).
It's still suboptimal, since the `n_sequence`s are per-input and not per-transaction,
so it should be possibile to spend different inputs with different, otherwise
incompatible, `CSV` timelocks, but that requires a larger refactor that
can be done in a future patch.
This commit also tries to clarify how the "policy path" should be used by adding
a fairly detailed example to the docs.
Previously `send_all` was particularly confusing, because when used on a
`create_tx` it implied two things:
- spend everything that's in the wallet (if no utxos are specified)
- don't create a change output
But when used on a `bump_fee` it only meant to not add a change output
and instead reduce the only existing output to increase the fee.
This has now been split into two separate options that should hopefully
make it more clear to use, as described in #142.
Additionally, `TxBuilder` now has a "context", that basically allows to
make some flags available only when they are actually meaningful, either
for `create_tx` or `bump_fee`.
Closes#142.
Thanks to the `ToWalletDescriptor` trait we can also very easily validate the checksum
for descriptors that are loaded from strings, if they contain one. Fixes#20.
