Sort mutations by table name before applying.
Mutations that span several files end up getting jumbled, preventing the

optimizer from truly living up to its potential.
Now, when pre-processing, we rearrange all the mutations to ensure

they're grouped together by table. This should generally be safe,

as mutations don't deal with foreign relations to other tables or

fields.
This allows the ALTER TABLE optimizer to do a better job of generating

fewer ALTER TABLE statements.

Unit tests pass.
Checked the output of the RB 1.7.x to 2.0 upgrade. It generated 13

table-rebuilding ALTER TABLE statements, across 13 tables.
For comparison, prior to all this work, there were 35 table-rebuilding

statements.