[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.

README.md

@@ -87,7 +87,7 @@ fn main() -> Result<(), bdk::Error> {
 ### Create a transaction
 
 ```rust,no_run
-use bdk::{FeeRate, TxBuilder, Wallet};
+use bdk::{FeeRate, Wallet};
 use bdk::database::MemoryDatabase;
 use bdk::blockchain::{noop_progress, ElectrumBlockchain};
 
@@ -108,12 +108,12 @@ fn main() -> Result<(), bdk::Error> {
     wallet.sync(noop_progress(), None)?;
 
     let send_to = wallet.get_new_address()?;
-    let (psbt, details) = wallet.create_tx(
-        TxBuilder::with_recipients(vec![(send_to.script_pubkey(), 50_000)])
-            .enable_rbf()
-            .do_not_spend_change()
-            .fee_rate(FeeRate::from_sat_per_vb(5.0))
-    )?;
+    let (psbt, details) = wallet.build_tx()
+        .add_recipient(send_to.script_pubkey(), 50_000)
+        .enable_rbf()
+        .do_not_spend_change()
+        .fee_rate(FeeRate::from_sat_per_vb(5.0))
+        .finish()?;
 
     println!("Transaction details: {:#?}", details);
     println!("Unsigned PSBT: {}", base64::encode(&serialize(&psbt)));