Avoid extremely long names in overloaded methods

[lucic71]

-   pub unsafe fn apply_Optionunsafefnpmuti32_i32__i32_const(
-        &self,
-        mut f: Option<unsafe fn(*mut i32, i32)>,
-        mut x: i32,
-    ) -> i32 {
+    pub unsafe fn apply_11(&self, mut f: Option<unsafe fn(*mut i32, i32)>, mut x: i32) -> i32 {

This generates more readable names and avoid special-casing characters in the mangled name.
Also it fixes a problem in the codegen where a template is visited before the type it's instantiated with.

[nunoplopes]

It would be great if the index could be skipped for non-overloaded methods.

[lucic71]

It would be great if the index could be skipped for non-overloaded methods.

It is, only overloaded get the index: https://github.com/Cpp2Rust/cpp2rust/pull/431/changes#diff-f772e2354c6403a95f445fd46043801b7f8128c9953f2fcf8b4860565d8c75e0R94