Add trait implementations for indexmap v2

[jonasbb]

Introduces a new feature flag indexmap_2 that enables the support.
Tests are copied from the v1 support.

Closes #621

[codecov]

Codecov Report

Merging #627 (f46b369) into master (20fae7c) will increase coverage by 0.24%.
Report is 2 commits behind head on master.
The diff coverage is 82.75%.

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
+ Coverage   64.22%   64.47%   +0.24%     
==========================================
  Files          36       36              
  Lines        2127     2156      +29     
==========================================
+ Hits         1366     1390      +24     
- Misses        761      766       +5     

Files Changed	Coverage Δ
serde_with/src/de/duplicates.rs	0.00% <ø> (ø)
serde_with/src/ser/impls.rs	91.66% <ø> (ø)
...e_with/src/duplicate_key_impls/first_value_wins.rs	79.41% <77.77%> (-0.59%)	⬇️
...with/src/duplicate_key_impls/error_on_duplicate.rs	86.36% <83.33%> (-1.14%)	⬇️
...de_with/src/duplicate_key_impls/last_value_wins.rs	86.36% <83.33%> (-1.14%)	⬇️
serde_with/src/de/impls.rs	58.83% <100.00%> (+0.16%)	⬆️

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[jonasbb]

bors r+

[bors]

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