add：SmallInteger()

[ProgrammerPlus1998]

The project is going to upgrade from the original sqlalchemy.MetaData() to ormar, but smallint cannot find the corresponding data type.
项目准备从原来的sqlalchemy.MetaData()升级为ormar，但是smallint找不到对应的数据类型。

[collerek]

Hi!
Thanks for the pr!
Can you also include some simple tests for this field?

[ProgrammerPlus1998]

I submitted code to an open source project for the first time. I didn't understand some of the rules and took some detours. Fortunately, it is basically solved now.
第一次给开源项目提交代码，不了解其中的一些规则，走了一些弯路，好在现在基本解决了。

[codecov-commenter]

Codecov Report

Merging #297 (0455595) into master (ef3edd4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #297   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          167       167           
  Lines        14237     14253   +16     
=========================================
+ Hits         14237     14253   +16     

Impacted Files	Coverage Δ
ormar/fields/__init__.py	100.00% <ø> (ø)
ormar/fields/model_fields.py	100.00% <100.00%> (ø)
tests/test_encryption/test_encrypted_columns.py	100.00% <100.00%> (ø)
tests/test_fastapi/test_choices_schema.py	100.00% <100.00%> (ø)
...sts/test_model_definition/test_model_definition.py	100.00% <100.00%> (ø)

[collerek]

@ProgrammerPlus1998 You did a great job for the first time! :)
Usually, you should have contributing guide either in GitHub or in documentation (like in ormar).
Not all projects have so strict rules for i.e coverage (like ormar has 100%).

Well done and thank you, will publish a new release in the nearest future to include those changes (and some bug fixes).
If you need this right now you can pip install from GitHub repo :)

Thanks again!