Use symbols instead of strings

ActiveSupport::Concern is used all over Rails
This PR will only create 3 new objects as keys are never recreated and
are not subject to garbage collection.

The strings were being uniquely created and garbage collected. I don't
have any performance numbers but this should be better than all of the
GC.
Making the rake file for tests easier to read.

I'm defining a new class which modularizes how the `rake test` tasks are
defined and invoked.
Improved grammar and replaced 'dbs' slang with 'databases'
eagerly initialize the attributes module to avoid check-then-set race conditions
initialize ivars so we don't have to constantly check them
Remove deprecated SchemaStatements#distinct
we don't need to use active support *everwhere* common.
define attribute methods in a thread safe manner
remove private attribute reader
lock around mutating the generated methods module
fix to_sql output on eager loaded relations
push attribute constant cache in to the attribute methods module
initialize generated modules on inclusion and on inheritence
eagerly assign the attribute name cache, remove const_missing
stop storing multiple copies of a particular attribute name
making the comment more accurate
freeze the constant value so it isn't duped when used as a hash key
reduce the amount of code Ruby has to parse
Remove the deprecated include_seconds argument from distance_of_time_in_words

Pass in an :include_seconds hash option to use this feature.
Remove deprecated constants autoload

These constants were removed in 4b97ce5eb16cc20207516387fba98bf577e2e281,
but I forgot to remove the autoload calls :sparkles:.