csview's trim_left and trim_right implicitly assume LTR text

[liigo]

see rust-lang/rust#30459

[tylov]

Thanks, liigo! Since these are new, I simply renamed them to trim_start() and trim_end() as suggested. (also matches with starts_with() and ends_with() functions).
I also renamed all the _right(s, len) functions to _tail(s, len) (because _end() is used for iterators).