Electrum update with outpoints and txids

[evanlinjin]

Related to #84

• Split bdk_electrum (library) out of bdk_electrum_example.
• Changed ElectrumClient logic to allowing syncing with arbitrary spks, outpoints, txids, full txs.
• Remove ElectrumError::Reorg variant and implement retry logic to mitigate reorgs.
• Introduced a bunch of convenience methods.
• Docs.

[codecov-commenter]

Codecov Report

Merging #191 (4eddf64) into master (3b26714) will not change coverage. The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #191   +/-   ##
=======================================
  Coverage   60.43%   60.43%           
=======================================
  Files           9        9           
  Lines         230      230           
=======================================
  Hits          139      139           
  Misses         91       91           

Impacted Files	Coverage Δ
bdk_chain/src/chain_graph.rs	81.81% <ø> (ø)
bdk_chain/src/sparse_chain.rs	77.77% <ø> (ø)

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[evanlinjin]

I've already released this! https://crates.io/crates/bdk_electrum