Remove cast to usize for BytePos and CharPos
Remove cast to usize for BytePos and CharPos

The case shouldn't be necessary and implicitly truncating BytePos is not
desirable.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust