feat: multi-column (tuple) expansion in In

[sonnemusk]

Summary

sqlx.In now expands slice-of-slices / slice-of-arrays as SQL tuples for multi-column IN lists (issue #294).

q, args, err := sqlx.In(
    `SELECT * FROM t WHERE (id, typ) IN (?)`,
    [][]interface{}{{1, "a"}, {2, "b"}},
)
// q:    SELECT * FROM t WHERE (id, typ) IN ((?, ?), (?, ?))
// args: []interface{}{1, "a", 2, "b"}

Also works with fixed-size rows:

sqlx.In(`WHERE (a, b) IN (?)`, [][2]int{{1, 2}, {3, 4}})

Rules

• Element type must be slice or array (not []byte — still a single bind value)
• All rows must share the same width
• Empty outer slice / empty tuple still errors as before

Fixes #294