The `UintError` type (returned by `read_uint_iter`) is not that useful
because one variant is unreachable. Re-write the function by doing:n
- Re-write the function to reduce the error cases returned.
- Re-name it to `read_push_data_len`
- Move it to `internals`
- Use `PushDataLenLen` enum instead of an int parameter
Tobin C. Harding