Slightly more efficient conversion from JSON to document

[asdine]

This PR is a small, low hanging, premature optimization.
It changes the way the document.NewFromJSON function works by lazily decoding the json data and delegating error handling to the Iterate and GetByField methods.
This takes advantage of the jsonparser library and prevents from duplicating the entire json object in memory before storing it.

It's 50% faster for small objects and about 20% for large objects, and 80% more memory efficient. It might be useful later for quick stream insertion.

name               old time/op    new time/op    delta
JSONToDocument-12    9.17µs ± 0%    7.86µs ± 0%   ~     (p=1.000 n=1+1)

name               old alloc/op   new alloc/op   delta
JSONToDocument-12    5.65kB ± 0%    3.10kB ± 0%   ~     (p=1.000 n=1+1)

name               old allocs/op  new allocs/op  delta
JSONToDocument-12       106 ± 0%       100 ± 0%   ~     (p=1.000 n=1+1)

[codecov-io]

Codecov Report

Merging #270 into master will increase coverage by 0.69%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
+ Coverage   61.35%   62.04%   +0.69%     
==========================================
  Files          68       68              
  Lines        6345     6450     +105     
==========================================
+ Hits         3893     4002     +109     
+ Misses       1936     1930       -6     
- Partials      516      518       +2     

Impacted Files	Coverage Δ
document/create.go	68.27% <72.72%> (+0.15%)	⬆️
database/table.go	57.25% <100.00%> (ø)
document/array.go	73.33% <100.00%> (+0.45%)	⬆️
document/document.go	75.93% <100.00%> (+1.06%)	⬆️
engine/memoryengine/store.go	91.96% <0.00%> (-1.79%)	⬇️
sql/parser/expr.go	84.10% <0.00%> (+6.29%)	⬆️

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[tie]

It looks like you din’t set the -count flag when running benchmarks (p=1.000 n=1+1).

[asdine]

10 times 👍🏼

name               old time/op    new time/op    delta
JSONToDocument-12    8.39µs ± 3%    7.06µs ± 2%  -15.82%  (p=0.000 n=10+9)

name               old alloc/op   new alloc/op   delta
JSONToDocument-12    5.65kB ± 0%    3.10kB ± 0%  -45.04%  (p=0.000 n=10+10)

name               old allocs/op  new allocs/op  delta
JSONToDocument-12       106 ± 0%       100 ± 0%   -5.66%  (p=0.000 n=10+10)