Improve `show` function of BUGSModel #955

Job	Run time
Julia 1 on ubuntu-latest (x64)	11m 15s
Julia 1 on macOS-latest (x64)	17m 18s
Julia 1 on windows-latest (x64)	15m 1s
Julia 1.9 on ubuntu-latest (x64)	24m 49s
Julia 1.9 on macOS-latest (x64)	23m 12s
Julia 1.9 on windows-latest (x64)	24m 40s
Julia nightly on ubuntu-latest (x64)	13m 52s
Julia nightly on macOS-latest (x64)	16m 21s
Julia nightly on windows-latest (x64)	18m 49s
	2h 45m 17s

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