Add static assertion ensuring sizeof(ExprEvalStep) <= 64 bytes

Add static assertion ensuring sizeof(ExprEvalStep) <= 64 bytes

This was previously only documented in a comment. Given the size of the
struct, it's not hard to miss that comment. As evidenced by the commits
leading up to fe3caa14393, 67b26703b41.

It's possible, but not likely, that we might have to weaken these assertions
on a less commonly used architecture.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/295606.1677101684@sss.pgh.pa.us