lang: Error parser if SetType fails to avoid a panic

Turns out we can actually cause the parser to error instead of needing
to panic. It definitely seems to work, and is better than the panic. The
only awkward thing is how this plumbing works in yacc world. If anyone
knows why this is wrong, please let me know. Reading the generated code
seems to imply that this is correct.

lang/lexparse_test.go

@@ -835,6 +835,30 @@ func TestLexParse0(t *testing.T) {
 			exp:  exp,
 		})
 	}
+	{
+		values = append(values, test{
+			name: "parser set type incompatibility str",
+			code: `
+			$x int = "hello"	# type should be str to work
+			test "t1" {
+				str => $x,
+			}
+			`,
+			fail: true,
+		})
+	}
+	{
+		values = append(values, test{
+			name: "parser set type incompatibility int",
+			code: `
+			$x int = "hello"	# value should be int to work
+			test "t1" {
+				int => $x,
+			}
+			`,
+			fail: true,
+		})
+	}
 
 	for index, test := range values { // run all the tests
 		name, code, fail, exp := test.name, test.code, test.fail, test.exp