Cleanup code for pre-1.53 versions

[jonasbb]

The MSRV got updated and this commit removes old code. Pre const-generic code could all be removed. Some further older entries could also be removed.

This makes rustversion a dev-dependency, since runtime changes are not necessary anymore.

Closes #451

bors merge

[codecov]

Codecov Report

Merging #453 (234a755) into master (4213291) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #453   +/-   ##
=======================================
  Coverage   71.87%   71.87%           
=======================================
  Files          54       54           
  Lines        3634     3634           
=======================================
  Hits         2612     2612           
  Misses       1022     1022           

Impacted Files	Coverage Δ
serde_with/src/de/mod.rs	83.33% <ø> (ø)
serde_with/src/hex.rs	76.92% <ø> (ø)
serde_with/src/lib.rs	75.00% <ø> (ø)
serde_with/src/ser/const_arrays.rs	81.25% <ø> (ø)
serde_with/src/ser/mod.rs	60.00% <ø> (ø)
serde_with/tests/serde_as/lib.rs	100.00% <ø> (ø)
serde_with/tests/serde_as/map_tuple_list.rs	100.00% <ø> (ø)

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[bors]

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