Make possible to specify subtype of SQL function

[gwenn]

Fix #1157
Breaking changes.

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (ccfbc28) 81.26% compared to head (f48c578) 81.34%.

Files	Patch %	Lines
src/functions.rs	94.73%	4 Missing ⚠️
src/pragma.rs	0.00%	4 Missing ⚠️
src/statement.rs	0.00%	4 Missing ⚠️
src/types/to_sql.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1160      +/-   ##
==========================================
+ Coverage   81.26%   81.34%   +0.08%     
==========================================
  Files          51       51              
  Lines       10345    10401      +56     
==========================================
+ Hits         8407     8461      +54     
- Misses       1938     1940       +2     

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