Implement Display+Error for NmeaError

[afonso360]

Hey! 👋

I'm trying to use this crate and have the errors propagated up. It looks like Error is not implemented for NmeaError, so here is a PR to fix that.

We implement Display unconditionally for NmeaError, and std::error::Errorwhen working in a std environment.

To distinguish between no_std and std we add an optional but enabled by default std feature. As far as I can tell, rust doesn't yet have a proper way to do this...

[codecov[bot]]

Codecov Report

Merging #35 (273cc54) into master (dcadb6c) will decrease coverage by 0.90%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   85.65%   84.75%   -0.91%     
==========================================
  Files          12       12              
  Lines         941      951      +10     
==========================================
  Hits          806      806              
- Misses        135      145      +10     

Impacted Files	Coverage Δ
src/parse.rs	71.92% <0.00%> (-15.31%)	:arrow_down:

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[Dushistov]

Thank you for PR.