This commit adds support for `UNION` in `internal/compiler`. It also adds
test cases for `UNION` in a `SELECT` and in a `WITH RECURSIVE` query.

It fixes sqlc-dev#568, sqlc-dev#723 and sqlc-dev#778.

…n-support

This commit adds enum values for SetOperation. The values are extracted from
the parser sources in pg_query_go:

typedef enum SetOperation
{
    SETOP_NONE = 0,
    SETOP_UNION,
    SETOP_INTERSECT,
    SETOP_EXCEPT
} SetOperation;