#1424: dup the String of Symbol#to_s in PTerm#to_p, so gsub! doesn't change a chilled String

[jack5505]

PTerm#to_p took the String of Symbol#to_s and changed it with gsub!. Since Ruby 3.4 that String is "chilled": the change still works, but prints string returned by :b1.to_s will be frozen in the future, and a later Ruby will raise FrozenError. Every variable name of every bylaw goes through this line, so with test.warning = true the build log was full of these warnings.

to_p now works on a copy, any.to_s.dup. Added a test that turns the deprecation warnings on, renders a bylaw and expects nothing on stderr; it fails without the fix.

Closes #1424