Remove some superlinear complexity

[brson]

Both unescape and ltrim were doing exponential work based on the number of matches in the input, by calling remove repeatedly, thus re-copying the same tail of the vector.

Surprisingly, I don't see a better safe tool for this than rotate_left offhand, which is a recently-stabilized API (and still isn't really the right tool), so I wrote some unsafe code, and only tested it lightly.

cc @SSJohns

[kivikakk]

LGTM. Unsafe code seems fairly foolproof. (famous last words.)

[brson]

Agh, copy_from is a 1.26 API too. I'll update it with an older function.

[kivikakk]

Are you depending on an earlier version? I figure targeting current stable (now 1.27) is reasonable given Rust's stable backwards compatibility.

[brson]

@kivikakk No I'm not, but comrak works with toolchains at least as far back as 1.23, and this doesn't seem worth breaking compatibility over.

[brson]

Now it's using ptr::copy.