nanonext 0.5.0

nanonext is now considered substantially feature-complete and API-stable

$context() method added for creating new contexts from nano Objects using supported protocols (i.e. req, rep, sub, surveyor, respondent) - this replaces the context() function for nano Objects.
subscribe() and unsubscribe() now accept a topic of any atomic type (not just character), allowing pub/sub to be used with integer, double, logical, complex, or raw vectors.
Sending via the "pub" protocol, the topic no longer needs to be separated from the rest of the message, allowing character scalars to be sent as well as vectors.
Added convenience auxiliary functions is_nano() and is_aio().

Protocol-specific helpers subscribe(), unsubscribe(), and survey_time() gain nanoContext methods.
Default protocol is now 'bus' when opening a new Socket or nano Object - the choices are ordered more logically.
Closing a stream now strips all attributes on the object rendering it a nil external pointer - this is for safety, eliminating a potential crash if attempting to re-use a closed stream.
For receives, if an error occurs in unserialisation or data conversion (e.g. mode was incorrectly specified), the received raw vector is now available at both $raw and $data if keep.raw = TRUE.
Setting 'NANONEXT_TLS=1' now allows the downloaded NNG library to be built against a system mbedtls installation.
Setting 'NANONEXT_ARM' is no longer required on platforms such as Raspberry Pi - the package configure script now detects platforms requiring the libatomic linker flag automatically.
Deprecated send_ctx(), recv_ctx() and logging removed.
All-round internal performance optimisations.

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