Fix deserialization of top-level sequences of enums

[Mingun]

Fixes #500. The behavior of top-level sequence actually very simple -- try to deserialize until Eof is received, which is the only stop marker.

After the fix in the first commit it became obvious, that some refactoring could be done to put highly coupled code together, which is done in the next 2 commits.

[codecov-commenter]

Codecov Report

Merging #501 (caab890) into master (0c6eeb0) will decrease coverage by 0.09%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
- Coverage   61.70%   61.61%   -0.10%     
==========================================
  Files          33       32       -1     
  Lines       15343    15307      -36     
==========================================
- Hits         9468     9432      -36     
  Misses       5875     5875              

Flag	Coverage Δ
unittests	61.61% <94.11%> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/de/mod.rs	67.16% <83.33%> (-0.07%)	⬇️
src/de/map.rs	71.36% <96.42%> (+3.29%)	⬆️
src/escapei.rs	12.35% <0.00%> (-0.18%)	⬇️
src/reader/mod.rs	92.04% <0.00%> (-0.12%)	⬇️

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