Adds a new type Masked that can mask data with a mask character, and can
be used anywhere we expect Cow<'a, str> or Text<'a>. E.g. Paragraph,
ListItem, Table Cells etc.
BREAKING CHANGE:
Because Masked implements From for Text<'a>, code that binds
Into<Text<'a>> without type annotations may no longer compile
(e.g. `Paragraph::new("".as_ref())`)
To fix this, annotate or call to_string() / to_owned() / as_str()