Add string concatenation support.

[whitequark]

This PR adds an infix operator ||. I didn't find any tests for specific
infix operators, so I didn't write any.

[ernie]

@whitequark This is db-specific -- better to use the existing support for infix operations.

[whitequark]

@ernie Could you elaborate? I believe the || operator is SQL-92, 4.2.2.1.

[ernie]

@whitequark it's SQL-92 but not supported by all db engines (read, LOL MYSQL)

[ernie]

In any case, since ARel already supports all manner of infix operations without this patch, I'm not sure I see the point in adding a specific implementation for concat.

[whitequark]

@ernie I see. Am I correct that you are suggesting to manually create the relevant nodes, and are opposed to the shortcut (like for arithmetic operations) because || is not supported by MySQL?

[ernie]

@whitequark Partly. Also because I'm not really sure concatenation is in the same category as the stuff in Arel::Math.

[whitequark]

@ernie Well I guess that counts as a reason. Feel free to close the PR or delegate it to an arel maintainer (not sure if you are one)

[ernie]

Sorry :(