"Customly parsed" SQL functions

[sbuberl]

Some SQL standard functions have special syntax to their parameters rather just comma separated. For example:

TRIM(LEADING 'x' FROM 'xxxbarxxx')
EXTRACT(YEAR_MONTH FROM '2009-07-02 01:02:03')

I did this before in the 1.4 rewrite branch. Just need to port them over.

[sbuberl]

Ported over most of the ones I did before. Added extract, overlay, position, and trim. And changed substring() to use standard syntax.

Only one I added before that is not ported is CAST() which I treat as function to simplify parsing. Will add CAST and see if any more I should add.

[sbuberl]

Basic version of CAST added which will expand after more stuff is added in future. Started adding some unit tests for this which I will finish before I close this out.

[sbuberl]

Done adding unit tests for all the functions. This is complete.