Improve type mismatch error messages

Replace "integral variable" with "integer" and replace
"floating-point variable" with "floating-point number" to make the
message less confusing.

src/test/ui/integral-variable-unification-error.rs

@@ -4,5 +4,5 @@ fn main() {
     //~^ ERROR mismatched types
     //~| expected type `{integer}`
     //~| found type `{float}`
-    //~| expected integral variable, found floating-point variable
+    //~| expected integer, found floating-point number
 }