Allow ?Sized implementations of Files for term::emit.

[ennis]

term::emit can work with ?Sized implementations of Files (e.g. dyn Trait), but it lacked the ?Sized bound.

[kaleidawave]

Not sure why CI did not run on first commit.

But I have locally run cargo check and found a small nit. Have committed it, CI has run this time.

CI now all green (for the sensible targets). Thanks for the change!