[wallet] Overhaul TxBuilder internals and externals

Fixes #251

TxBuilders are now not created directly but are created through the
wallet with `build_tx` and `build_fee_bump`.
The advantages of this realised in this commit are:

1. Normal tx creation and fee bumping use the code internally. The only
difference between normal tx and fee bump is how the builder is created.
2. The TxBuilder now has a refernce to the wallet and can therefore
lookup things as methods are called on it. `add_utxo` now uses this to
look up UTXO deta when it is called (rather than having to do it and
possibly error later on).

To support these changes `get_utxo` and `get_descriptor_for_keychain`
public methods have been added to Wallet. I could have kept them
pub(crate) but they seem like fine APIs to have publicly.

src/wallet/address_validator.rs

@@ -127,7 +127,6 @@ mod test {
 
     use super::*;
     use crate::wallet::test::{get_funded_wallet, get_test_wpkh};
-    use crate::wallet::TxBuilder;
 
     struct TestValidator;
     impl AddressValidator for TestValidator {
@@ -158,10 +157,9 @@ mod test {
 
         let addr = testutils!(@external descriptors, 10);
         wallet
-            .create_tx(TxBuilder::with_recipients(vec![(
-                addr.script_pubkey(),
-                25_000,
-            )]))
+            .build_tx()
+            .add_recipient(addr.script_pubkey(), 25_000)
+            .finish()
             .unwrap();
     }
 }