Most char API should use Copy instead of Ref

[Behnam Esfahbod]

Most functions taking char as input, specially those taking self with a small struct only having one or two char, should be migrated to take self instead of &self.

Some of the clippy warnings:

warning: this argument is passed by reference, but would be more efficient if passed by value
   --> unic/char/range/src/range.rs:180:21
    |
180 |     pub fn contains(&self, ch: char) -> bool {
    |                     ^^^^^ help: consider passing by value instead: `self`
    |
    = note: #[warn(clippy::trivially_copy_pass_by_ref)] on by default
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#trivially_copy_pass_by_ref

warning: this argument is passed by reference, but would be more efficient if passed by value
   --> unic/char/range/src/range.rs:190:21
    |
190 |     pub fn cmp_char(&self, ch: char) -> Ordering {
    |                     ^^^^^ help: consider passing by value instead: `self`
    |
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#trivially_copy_pass_by_ref

warning: this argument is passed by reference, but would be more efficient if passed by value
   --> unic/char/range/src/range.rs:203:16
    |
203 |     pub fn len(&self) -> usize {
    |                ^^^^^ help: consider passing by value instead: `self`
    |
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#trivially_copy_pass_by_ref

warning: this argument is passed by reference, but would be more efficient if passed by value
   --> unic/char/range/src/range.rs:208:21
    |
208 |     pub fn is_empty(&self) -> bool {
    |                     ^^^^^ help: consider passing by value instead: `self`
    |
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#trivially_copy_pass_by_ref

warning: this argument is passed by reference, but would be more efficient if passed by value
   --> unic/char/range/src/range.rs:213:17
    |
213 |     pub fn iter(&self) -> CharIter {
    |                 ^^^^^ help: consider passing by value instead: `self`
    |
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#trivially_copy_pass_by_ref