implementation must:
</p>

<ul>
	<li>Represent integer constants with at least 256 bits.</li>

	<li>Represent floating-point constants, including the parts of
	    a complex constant, with a mantissa of at least 256 bits
	    and a signed binary exponent of at least 16 bits.</li>

	<li>Give an error if unable to represent an integer constant
	    precisely.</li>

		//       var a [3]int
		//       p := (*[2]int)(a[:])
		//       q := (*[2]int)(a[1:])
		//       *p = *q
		// We don't care about solving 1. Or at least, we haven't historically
		// and no one has complained.
		// For 2, we need to ensure that if there might be partial overlap,
		// then we can't use OpMove; we must use memmove instead.