Support type overrides for parameter structs

[kyleconroy]

Given the following tables and queries, type overrides should apply to the inputs of GetFoo and GetBar.

CREATE TABLE foo (id bigserial not null);
CREATE TABLE bar (id bigserial not null);

-- name: GetFoo :one
SELECT * FROM foo WHERE id = $1;

-- name: GetBar :one
SELECT * FROM bar WHERE id = $1;

Originally posted by @tv42 in #607