Rename Lrama::Grammar::Rule#to_s to Lrama::Grammar:Rule::display_name #1278

Job	Run time
test-memory (head)	58s
check-misc (head)	13s
steep-check (head)	13s
test-windows (head)	2m 30s
test-ruby (head, master)	5m 27s
test-ruby (3.0, master)	5m 20s
ruby-versions / Generate Ruby versions	7s
test (2.5)	23s
test (2.6)	23s
test (2.7)	24s
test (3.0)	22s
test (3.1)	16s
test (3.2)	10s
test (3.3)	12s
test (head)	16s
	17m 14s

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