add options to binary_to_term

term_to_binary and binary_to_term are powerful tools that can be used easily in
lieu of a custom binary network protocol.  Unfortunately, carefully crafted
data can be used to exhaust the memory in an Erlang node by merely attempting
to decode binaries.  This makes it unsafe to receive data from untrusted
sources.

This is possible because binary_to_term/1 will allocate new atoms and new
external function references.  These data structures are not garbage collected.

This patch implements the new form of binary_to_term that takes a list of
options, and a simple option called 'safe'.  If specified, this option will
cause decoding to fail with a badarg error if an atom or external function
reference would be allocated.

In the general case, it will happily decode any Erlang term other than those
containing new atoms or new external function references.  However, fun, pid,
and ref data types can embed atoms.  They might fail to decode if one of these
embedded atoms is new to the node.  This may be an issue if encoded binaries
are transferred between nodes or persisted between invocations of Erlang.

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