Unicode: `.calculate_width` raising on UTF-8 valid characters

## Description

Since the last release, when using IRB we noticed a `TypeError` was being raised instead of the result being printed. I traced back the issue to `Redline::Unicode.calculate_width`
The minimal code to reproduce the error is 
```ruby
Reline::Unicode.calculate_width("é")
```
Happened on both Ruby 2.6.6 & 2.7.1

## Terminul Emulator

I'm using iTerm on Mac