The old code generated always generated a constraint of the form
col = ANY(ARRAY[val1, val2, ...]), but that's invalid when col is an
array type.  Instead, generate col = val when there's only one value,
col = val1 OR col = val2 OR ... when there are multiple values and
col is of array type, and the old form when there are multiple values
and col is not of an array type.

As a side benefit, this makes constraint exclusion able to prune
a list partition declared to accept a single Boolean value, which
didn't work before.

Amit Langote, reviewed by Etsuro Fujita

Discussion: http://postgr.es/m/97267195-e235-89d1-a41a-c110198dfce9@lab.ntt.co.jp