Support IntoIterator<Item = String> in flamegraph::from_lines

[itamarst]

Right now this API takes IntoIterator<Item = &'a str> and it's a pain to turn IntoIterator<Item = String> into the former. Googling suggests the solution is accepting IntoIterator<Item = AsRef<str>>, which should be backwards compatible?

[jonhoo]

Unfortunately this is harder than it sounds — we specifically need to get string references with a 'a lifetime in the methods in question so that we don't have to re-allocate and copy every string argument we're given. Notice how the code also uses 'a for the other arguments here:

inferno/src/flamegraph/merge.rs

Lines 28 to 29 in 74f2fb0

	tmp: &mut HashMap<Frame<'a>, FrameTime>,
	frames: &mut Vec<TimedFrame<'a>>,

It would be nice to make the API work in such a way that the whole input doesn't need to be kept allocated across the runtime of the analysis, but it would require some deeper changes I think.