Version 0.2.0

• Added EcoString::make_mut for getting an &mut str
• Added EcoString::{replace, replacen, to_ascii_lowercase, to_ascii_uppercase} which perform the respective operation and return an EcoString instead of a String. (Breaking change)
• Added missing #[inline] annotation (thanks to @Kmeakin)
• Fixed typos and marked one internal function unsafe (thanks to @hwjsnc)