Send scheme procedure as callback to foreign thread.

[Zhu Zihao]

In Guile FFI programming, we have procedure->pointer, which makes a
Scheme procedure a foreign callback. And foreign library call use this
callback.

However, if this callback is called in a another foreign thread. The scheme
context is not properly setup, It may cause crash or UB.

IIUC, the procedure invoker "invoke_closure" defined in foreign.c
doesn't use scm_with_guile. If we use scm_with_guile to make a foreign
callback, would it be OK to execute Scheme procedure in multithreading context?
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Zihao

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