We set up DATE and TIMESTAMP differently in field-creation than we
did in field-MD creation (for CREATE). Admirably, ALTER TABLE
detected this and didn't damage any data, but it did initiate a
full copy/conversion, which we don't really need to do.

Now we describe Field and Create_field the same for those types.
As a result, ALTER TABLE that only changes meta-data (like a
field's name) no longer forces a data-copy when there needn't
be one.


mysql-test/r/alter_table.result:
  0 rows should be affected when a meta-data change is enough ALTER TABLE.
mysql-test/t/alter_table.test:
  add test-case: show that we don't do a full data-copy on ALTER TABLE
  when we don't need to.
sql/field.cc:
  Remove Field_str::compare_str_field_flags() (now in Field/Create_field as
  field_flags_are_binary().
  
  Correct some field-lengths!
sql/field.h:
  Clean-up: use defined constants rather than numeric literals for certain
  field-lengths.
  
  Add enquiry-functions binaryp() to classes Field and Create_field.
  This replaces field.cc's Field_str::compare_str_field_flags().