Refactor wildcard expressions

[alanmcruickshank]

Fixes #449 . This introduces a backward incompatible change to the handling of * segments but I think it's for the best.

[codecov]

Codecov Report

Merging #454 into master will increase coverage by 0.01%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
+ Coverage   91.73%   91.74%   +0.01%     
==========================================
  Files          30       30              
  Lines        4633     4654      +21     
==========================================
+ Hits         4250     4270      +20     
- Misses        383      384       +1     

Flag	Coverage Δ
#py35	91.61% <97.36%> (+0.01%)	⬆️
#py36	91.61% <97.36%> (+0.01%)	⬆️
#py37	91.61% <97.36%> (+0.01%)	⬆️
#py38	91.56% <97.43%> (+0.01%)	⬆️
#py39	91.56% <97.43%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/sqlfluff/rules/std.py	93.67% <87.50%> (-0.15%)	⬇️
src/sqlfluff/dialects/dialect_ansi.py	99.64% <100.00%> (+0.02%)	⬆️
src/sqlfluff/dialects/dialect_bigquery.py	100.00% <100.00%> (ø)
src/sqlfluff/parser/segments_base.py	88.21% <100.00%> (+0.04%)	⬆️

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