Newtype

[jprochazk]

Closes #50

Adds the #[garde(transparent)] attribute, which may be used to create easily re-usable newtypes:

#[derive(garde::Validate)]
#[garde(transparent)]
struct Username(#[garde(length(min = 3, max = 20))] String);

#[derive(garde::Validate)]
struct User {
    #[garde(dive)]
    username: Username,
}

#[garde(transparent)] is only allowed on tuples and structs with exactly one field. It will flatten the error path by one level, which means that User.username[0] will be just User.username instead, resulting in nicer error messages:

User {
  username: Username("")
}

username: length is lower than 3