improve ledger initialization for fullnode

[rob-solana]

• use a line iterator on stdin instead of a line iterator on a buffer
• move some unwrap() to expect(), documenting failures
• bind entry type earlier (for kicks)

[codecov]

Codecov Report

Merging #380 into master will increase coverage by 0.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   90.78%   91.06%   +0.27%     
==========================================
  Files          36       36              
  Lines        3493     3490       -3     
==========================================
+ Hits         3171     3178       +7     
+ Misses        322      312      -10

Impacted Files	Coverage Δ
src/entry.rs	98.85% <0%> (-0.04%)	⬇️
src/crdt.rs	85.36% <0%> (+1.15%)	⬆️
src/tvu.rs	99.27% <0%> (+1.45%)	⬆️

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