Pre-process mutations to optimize table modifications.
Database mutations are now pre-processed in order to filter out

mutations that ultimately have no effect on the database, or those that

can be easily combined into earlier mutations.
Lists of mutations are first divided up into batches of mutations that

can be considered together (AddField, ChangeField, RenameField,

DeleteField, basically). Each batch is then processed for a number of

optimizations.
The optimizations include:


Removing all mutations for a batch that both adds and deletes a

  particular field, which would otherwise result in an expensive

  no-op on the table.


Removing all mutations operating on a field that's later deleted in

  a DeleteField.


Condensing multiple ChangeFields in a batch together into one

ChangeField.


Condensing all changes made by ChangeFields for a field into the

AddField that introduced the field, if an AddField is present in the

  batch.


Condensing multiple RenameFields in a batch together into a single

RenameField.


Removing all RenameFields and changing the field name for an AddField

  when both are present in a batch.


This can reduce the time needed for evolutions by a fair amount,

particularly when upgrading an older database with many changes pending.

Introduced a bunch of unit tests, which all pass.
Ran against a RB 1.7.x database. It evolved, and the number of

ALTER TABLE statements was reduced by 5.