This fixes a compilation warning with newer clang version.
Indeed, the whitespace between "" and _bitlength is no longer
allowed. This was permitted in older versions of the standard but has
since been deprecated.
```
error: identifier '_bitlength' preceded by whitespace in a literal
operator declaration is deprecated
[-Werror,-Wdeprecated-literal-operator]
27 | constexpr uint8_t operator "" _bitlength()
| ~~~~~~~~~~~~^~~~~~~~~~
| operator""_bitlength
```
