Previously GC could trigger a table rebuild of the generic fields st_table in the middle of calling the st_update callback. This could cause entries to be reallocated or rearranged and the update to be for the wrong entry.

This commit adds an assertion to make that case easier to detect, and replaces the st_update with a separate st_lookup and st_insert.

This should be an additional fix to the issue we've been seeing in test_array.rb in CI (which has also had another workaround).

We used this as a (somewhat) reliable reproduction:

objs = 10_000.times.map do
  a = []
  a.instance_variable_set(:@a, 1)
  a
end

GC.stress = true
GC.auto_compact = true

steps = 1000.times.map do
  a = []
  a.instance_variable_set(:@a, 1)
  a.instance_variable_set(:@b, 1)
  a.instance_variable_set(:@c, 1)
  a.instance_variable_set(:@d, 1)
  a.instance_variable_set(:@e, 1)
  a
end

GC.stress = false
GC.auto_compact = false