Txgraph fixes

[LLFourn]

• We were recording spends of coinbase tx. Fixed and added tests.
• removed is_unspent to spent_by which returns the actual spender and index in chain.

[codecov-commenter]

Codecov Report

Merging #69 (b02edd4) into master (93a5e0e) will increase coverage by 5.12%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   43.87%   49.00%   +5.12%     
==========================================
  Files           9        9              
  Lines         604      602       -2     
==========================================
+ Hits          265      295      +30     
+ Misses        339      307      -32     

Impacted Files	Coverage Δ
bdk_core/src/chain_data.rs	43.24% <ø> (ø)
bdk_core/src/chain_graph.rs	0.00% <ø> (ø)
bdk_core/src/sparse_chain.rs	78.57% <ø> (ø)
bdk_core/src/tx_graph.rs	54.01% <71.42%> (+22.35%)	:arrow_up:

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