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 fe3caa1439, 67b26703b4.

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

src/include/executor/execExpr.h

@@ -669,6 +669,10 @@ typedef struct ExprEvalStep
 	}			d;
 } ExprEvalStep;
 
+/* Enforce the size rule given in the comment above */
+StaticAssertDecl(sizeof(ExprEvalStep) <= 64,
+				 "size of ExprEvalStep exceeds 64 bytes");
+
 
 /* Non-inline data for container operations */
 typedef struct SubscriptingRefState