Annotate Symbol#to_s as leaf

[k0kubun]

A leaf annotation allows the interpreter and YJIT to call the method without pushing a frame.

It seems to speed up railsbench a little:

before: ruby 3.4.0dev (2024-01-30T19:59:53Z master c1f8d974a8) +YJIT [x86_64-linux]
after: ruby 3.4.0dev (2024-01-30T21:33:27Z symbol-to-s b49e185cb6) +YJIT [x86_64-linux]

----------  -----------  ----------  ----------  ----------  -------------  ------------
bench       before (ms)  stddev (%)  after (ms)  stddev (%)  after 1st itr  before/after
railsbench  2473.4       0.9         2455.6      0.6         0.99           1.01
----------  -----------  ----------  ----------  ----------  -------------  ------------