builtin: optimise `sx == sy` in the case where strings have common prefixes

vlib/builtin/string.v

@@ -496,6 +496,7 @@ pub fn (s string) u64() u64 {
 }
 
 // eq implements the `s == a` (equal) operator.
+[direct_array_access]
 fn (s string) eq(a string) bool {
 	if s.str == 0 {
 		// should never happen
@@ -504,6 +505,12 @@ fn (s string) eq(a string) bool {
 	if s.len != a.len {
 		return false
 	}
+	if s.len > 0 {
+		last_idx := s.len - 1
+		if s[last_idx] != a[last_idx] {
+			return false
+		}
+	}
 	unsafe {
 		return C.memcmp(s.str, a.str, a.len) == 0
 	}