Overhaul of Builder APIs.

[Anders429]

Fixes #17.

This fixes a few things in the de::Builder API. Firstly, this makes it impossible for build() to panic, instead requiring that Builder is always in a valid state (meaning tokens is always set). Additionally, the internals of Builder are refactored to no longer use any Options, as well as collecting the tokens preemptively to prevent having to clone an iterator.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (4d3d701) 98.23% compared to head (4a616e2) 98.18%.
Report is 1 commits behind head on dev.

Files	Patch %	Lines
src/ser.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #26      +/-   ##
==========================================
- Coverage   98.23%   98.18%   -0.05%     
==========================================
  Files           3        3              
  Lines        6569     6446     -123     
==========================================
- Hits         6453     6329     -124     
- Misses        116      117       +1     

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[Anders429]

Also reworked ser::Builder in a similar way, removing the Options.