Constant vector length should accept non-literal constants

[jdm]

rusti> const foo: uint = 8;
rusti> const bar: [uint * foo] = [0, 1,2,3,4,5,6,7];
<anon>:37:19: 37:22 error: expected integral vector length but found `foo`

[sanxiyn]

This is #3469.