Rollup merge of #143552 - nagisa:makes-ceil-char-boundary-go-zoom, r=jhpratt

lib: more eagerly return `self.len()` from `ceil_char_boundary`

There is no reason to go through the complicated branch as it would
always return `self.len()` in this case. Also helps debug code somewhat
and I guess might make optimizations easier (although I haven't really a
sample to demonstrate this.)

ref. rust-lang/rust#93743
Suggested by `@chrisduerr`

Author: jhpratt