Everywhere: Use to_number<T> instead of to_{int,uint,float,double}

In a bunch of cases, this actually ends up simplifying the code as
to_number will handle something such as:

```
Optional<I> opt;
if constexpr (IsSigned<I>)
    opt = view.to_int<I>();
else
    opt = view.to_uint<I>();
```

For us.

The main goal here however is to have a single generic number conversion
API between all of the String classes.

Userland/Libraries/LibHTTP/HttpRequest.cpp

@@ -183,7 +183,7 @@ ErrorOr<HttpRequest, HttpRequest::ParseError> HttpRequest::from_raw_request(Read
                 commit_and_advance_to(current_header.value, next_state);
 
                 if (current_header.name.equals_ignoring_ascii_case("Content-Length"sv))
-                    content_length = current_header.value.to_uint();
+                    content_length = current_header.value.to_number<unsigned>();
 
                 headers.append(move(current_header));
                 break;