Add EcoString::truncate

[ronnodas]

Should behave like String::truncate, ie panic if new_len not on a char boundary. The "same" implementation should work too, although I was expecting DynamicVec::truncate to move from Spilled to Inline if new_len <= 15 but it doesn't seem to do that. I can try making a PR for one or both of these things if they sound like improvements.

[laurmaedje]

I was expecting DynamicVec::truncate to move from Spilled to Inline if new_len <= 15 but it doesn't seem to do that.

It doesn't do that because you might go above and below the limit frequently and then it would have a ton of allocations. Works the same way in smallvec.

I'm open for a PR that adds a EcoString::truncate behaving just like std's.