ui: Add `Session::set_character()` and a fluff forwarding test. #188

Job	Run time
miri	2m 33s
no_std	39s
build (ubuntu, stable, locked, true)	58m 50s
build (windows, stable, locked, false)	28m 53s
build (macos, stable, locked, false)	22m 5s
build (ubuntu, nightly, locked, false)	16m 26s
build (ubuntu, beta, locked, false)	17m 49s
build (ubuntu, stable, latest, false)	27m 28s
build (ubuntu, stable, minimal, false)	29m 5s
deploy	9s
fuzz	17m 20s
	3h 41m 17s

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