Remove needless collect to Vec in JsonMapTrait<'json, json::JsonValue>

[macisamuele]

PR #7 relaxed the constraint associated to the iterator types returned by JsonMapTrait.
Thanks to it we don't actually need to have .collect().into_iter() which actually implies:

• needless memory allocation
• processing time (to duplicate materialise the items into the new memory location)

Even if we would not have relaxed the iterator type we might have had the possibility to remove it as json>=0.11.14 does return ExactSizeIterator ;)

[codecov]

Codecov Report

Merging #11 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #11   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files           8        8           
  Lines         687      687           
=======================================
  Hits          654      654           
  Misses         33       33

Impacted Files	Coverage Δ
src/traits/_json.rs	98.09% <100%> (ø)	⬆️

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