Allow symbols used for scopes to move

[peterzhu2118]

This commit allows symbols used for instance variables, global variables, constants, class names to be moved by GC compaction.

For example:

require "objspace"

Object.const_set("Hello".to_sym, 1)
sym = "Hello".to_sym

puts ObjectSpace.dump(sym)
GC.verify_compaction_references(expand_heap: true, toward: :empty)
puts ObjectSpace.dump(sym)

Before:

{"address":"0x101274ac8", "type":"SYMBOL", ... }
{"address":"0x101274ac8", "type":"SYMBOL", ... }

After:

{"address":"0x101544a78", "type":"SYMBOL", ... }
{"address":"0x1015d7f80", "type":"SYMBOL", ... }