Add support for nullable items of maps and slices

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Lines Of Code

Language	Files	Lines	Code	Comments	Blanks	Complexity	Bytes
Go	6	1337	948	114	275	388	30.5K (-38B)
Go (test)	7	1273 (+23)	969 (+20)	125	179 (+3)	17	33.6K (+542B)
YAML	6	359 (-1)	312 (-1)	31	16	0	11.4K (-14B)

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Exported API Changes Report

# summary
Inferred base version: v0.3.20
Suggested version: v0.3.21

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Unit Test Coverage

total: (statements) 70.1%

Coverage diff with base branch

```diff ```

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Codecov Report

Merging #22 (d39c5cc) into master (db23b28) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   64.68%   64.68%           
=======================================
  Files           6        6           
  Lines         875      875           
=======================================
  Hits          566      566           
  Misses        251      251           
  Partials       58       58           

Flag	Coverage Δ
unittests	64.68% <100.00%> (ø)

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

Impacted Files	Coverage Δ
reflect.go	72.30% <100.00%> (ø)

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Benchmark Result

Benchmark diff with base branch

``` name old time/op new time/op delta Schema_UnmarshalJSON_raw-2 100µs ±15% 97µs ±11% ~ (p=0.421 n=5+5) Schema_UnmarshalJSON-2 742µs ± 2% 784µs ± 7% ~ (p=0.095 n=5+5) Schema_MarshalJSON_raw-2 106µs ± 7% 113µs ± 6% ~ (p=0.095 n=5+5) Schema_MarshalJSON-2 323µs ± 5% 340µs ± 6% ~ (p=0.056 n=5+5) name old alloc/op new alloc/op delta Schema_UnmarshalJSON_raw-2 31.7kB ± 0% 31.7kB ± 0% ~ (p=0.175 n=5+5) Schema_UnmarshalJSON-2 187kB ± 0% 187kB ± 0% ~ (p=0.556 n=4+5) Schema_MarshalJSON_raw-2 28.8kB ± 0% 28.8kB ± 0% ~ (p=1.000 n=5+4) Schema_MarshalJSON-2 53.8kB ± 0% 53.8kB ± 0% ~ (p=0.635 n=5+5) name old allocs/op new allocs/op delta Schema_UnmarshalJSON_raw-2 457 ± 0% 457 ± 0% ~ (all equal) Schema_UnmarshalJSON-2 1.94k ± 0% 1.94k ± 0% ~ (all equal) Schema_MarshalJSON_raw-2 621 ± 0% 621 ± 0% ~ (all equal) Schema_MarshalJSON-2 484 ± 0% 484 ± 0% ~ (all equal) ```