Redesign line reader

[ZoeyR]

The current line reader was horribly broken. The redesign makes the
reader match the datasheet more closely. Insead of reading until \r\n
and returning all text up until then, the new reader finds the leading
\r\n that preceeds all sim7000 lines. It then reads up to the following
\r\n to terminate the line. This resolves intermittent issues with echo
as well as some unsolicited result codes.

[codecov]

Codecov Report

Merging #9 (8566e4c) into main (1dfaae3) will increase coverage by 1.50%.
The diff coverage is 67.14%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   35.02%   36.52%   +1.50%     
==========================================
  Files          32       32              
  Lines         748      720      -28     
==========================================
+ Hits          262      263       +1     
+ Misses        486      457      -29     

Impacted Files	Coverage Δ
src/commands/ccid.rs	0.00% <ø> (ø)
src/commands/cgmr.rs	0.00% <ø> (ø)
src/commands/cgnscpy.rs	0.00% <ø> (ø)
src/commands/cgnsinf.rs	52.94% <ø> (+1.99%)	⬆️
src/commands/cgnspwr.rs	0.00% <ø> (ø)
src/commands/cgreg.rs	0.00% <ø> (ø)
src/commands/cipstart.rs	0.00% <0.00%> (ø)
src/commands/cipstatus.rs	0.00% <0.00%> (ø)
src/commands/cntp.rs	0.00% <ø> (ø)
src/commands/cops.rs	0.00% <ø> (ø)
... and 7 more

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