Add support for aliases

[jeffzi]

This PR adds support for aliases in SchemaModel and closes #324.

Example:

    import pandera as pa
    import pandas as pd

    class Schema(pa.SchemaModel):
        col_2020: pa.typing.Series[int] = pa.Field(alias=2020)
        idx: pa.typing.Index[int] = pa.Field(alias="_idx", check_name=True)

        @pa.check(2020)
        def int_column_lt_100(cls, series):
            return series < 100


    df = pd.DataFrame({2020: [99]}, index=[0])
    df.index.name = "_idx"

    Schema.validate(df)

I also restructured the schema construction. I replaced the internal attribute __fields_annotations__ by __fields__ which is a dict indexed by public column/index name (i.e. optionally aliased) and values are tuples of (AnnotationInfo, FieldInfo). That gives us more flexibility.

[codecov-io]

Codecov Report

Merging #329 (58472dd) into master (ac41212) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #329   +/-   ##
=======================================
  Coverage   98.79%   98.80%           
=======================================
  Files          18       18           
  Lines        1747     1750    +3     
=======================================
+ Hits         1726     1729    +3     
  Misses         21       21           

Impacted Files	Coverage Δ
pandera/decorators.py	100.00% <100.00%> (ø)
pandera/model.py	100.00% <100.00%> (ø)
pandera/model_components.py	100.00% <100.00%> (ø)
pandera/typing.py	100.00% <100.00%> (ø)

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[cosmicBboy]

🔥 thanks @jeffzi! this functionality keeps getting better and better :)