Create real array comparison functions (that use the element datatype's

Create real array comparison functions (that use the element datatype's
comparison functions), replacing the highly bogus bitwise array_eq.  Create
a btree index opclass for ANYARRAY --- it is now possible to create indexes
on array columns.
Arrange to cache the results of catalog lookups across multiple array
operations, instead of repeating the lookups on every call.
Add string_to_array and array_to_string functions.
Remove singleton_array, array_accum, array_assign, and array_subscript
functions, since these were for proof-of-concept and not intended to become
supported functions.
Minor adjustments to behavior in some corner cases with empty or
zero-dimensional arrays.

Joe Conway (with some editorializing by Tom Lane).