Fix #290, #383, #407 where APC_EXTENSIBLE is lost for SEQUENCE SIZE(, ...) OF

* check if __EXTENSIONS__ if alreay
  defined

The encode function call will crash if it's not initialized. Update the document to make it more clear.

    In some cases an INTEGER overflow during parsing is not detected
    and incorrect value is returned to the decoder instead of an error.
    Reported by Nika Pona <npona@digamma.ai>.
    (Severity: low; Seuciry impact: medium).

The function `SET_OF__encode_sorted` stored the working element buffer
in a static variable. If called recursively (that is, when performing
encoding of a SET containing another SET) the outermost element buffer
is therefore clobbered by the innermost call.

This causes a SEGV on continuation of the outermost call.

Bug found during testing of [https://github.com/surevine/spiffing Spiffing],
fix verified with the same test suite.

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