Pg: Add escape_literal() and escape_identifier()

Didn't expect I'd ever need these, but they're useful for generating SQL
scripts.

t/pgconnect.t

@@ -353,6 +353,16 @@ subtest 'txn', sub {
     is_deeply $st->val, [1,3], 'not deep copy';
 }
 
+
+{
+    # Exact format returned by escape_literal() can differ between Postgres versions and configurations.
+    my $x = q{"' \" \\};
+    is $conn->q('SELECT '.$conn->escape_literal($x))->val, $x;
+
+    # Format can also change, but unsure how to test this otherwise.
+    is $conn->escape_identifier('hel\l"o'), '"hel\l""o"';
+}
+
 subtest 'Prepared statement cache', sub {
     my $txn = $conn->cache_size(2)->txn->cache;
     my sub numexec($sql) {