This patch corrects two long-standing bugs in CockroachDB which were
confusing both 3rd party tools/ORMs and users:

- the SQL standard type called "CHAR" is supposed to have a default
  maximum width of 1 character.

  Prior to this patch, CockroachDB did not support this and instead
  set no default maximum width on CHAR. This would cause CockroachDB
  to fail validating the width of values inserted in tables, fail
  to constrain the width of values during casts to CHAR, and report
  incorrect information in introspection tables.

- PostgresSQL's dialect distinguishes the table column types TEXT,
  CHAR, VARCHAR and a special thing called `"char"` (this is a legacy
  PostgreSQL type which is, unfortunately, still used by some
  pg_catalog tables).

  Prior to this patch, CockroachDB would map all of these types into
  the same column type "STRING". In turn this caused them to show
  up as "text" in introspection.

  While this was not incorrect from the perspective of value
  handling (all these types are, indeed, handled with about the same
  value semantics in both PostgreSQL and CockroachDB), ORMs
  do pay attention to this distinction and become confused if they
  see a different type in introspection (e.g. "text") than what
  the application model requires (e.g. "char"). The result of this
  confusion was to trigger a schema change to adapt the type, which
  in turn fails due to missing features in ALTER COLUMN TYPE.

This patch corrects both problems by giving the appropriate default
width to CHAR and preserving the distinction between the various string
types in column descriptors and thus introspection.

Additionally, a width constraint check on collated string columns was
missing. This is now fixed too.

Tables and columns created prior to this patch are unaffected.

Release note (bug fix): CockroachDB now distinguishes "CHAR" and
"VARCHAR" as mandated by the SQL standard and PostgreSQL
compatibility. When a width is not specified (e.g. `CHAR(3)`), the
maximum width of `VARCHAR` remains unconstrained whereas the maximum
width for `CHAR` is now 1 character.

Release note (bug fix): CockroachDB now properly checks the width of
strings inserted in a collated string column with a specified width.

Release note (sql change): CockroachDB now preserves the distinction
between different column types for string values like in PostgreSQL,
for compatibility with 3rd party tools and ORMs.