[CM] Possible bug with c-object & openlet

[bil at ccrma.Stanford.EDU]

You need to gc protect the methods -- c_object_set_let
does not do that.  This is partly for speed and to save
space in the gc-protected-objects list, and also because
you might create the let holding the methods, then
any amount of time might go by before you make an
object, so the let has to be gc-protected anyway.
g_block_methods in s7test.scm is an example where
each block can be considered an instance of the
g_block class, and the let is protected once.