Renamed README -> README.org; fixed and extended content

* README.org: fixed some typos; extended meta-data; added section
  about sending messages
Use let-plus system instead of metabang-bind system

* src/connection.lisp (receive :around connection): use `let+' instead
  of `bind'
  (receive connection): likewise
  (connect string): likewise
* src/package.lisp (package network.spread): changed used package
  metabang-bind -> let-plus
* test/connection.lisp (test connection-root::message-size-limit): use
  `flet' instead of `bind'
* test/package.lisp (system network.spread.test): changed used package
  metabang-bind -> let-plus
* network.spread.asd (system network.spread): changed system
  dependency metabang-bind -> let-plus
Removed *{incoming,outgoing}-stream* in src/variables.lisp

* src/variables.lisp (*incoming-stream*): removed; has not been used
  anywhere
  (*outgoing-stream*): likewise
* src/connection.lisp (receive :around connection):
  removed; *incoming-stream* has been removed
  (send-bytes :before connection): changed qualifier :around ->
  :before; removed code related to *outgoing-stream*
* src/package.lisp (package network.spread): removed exported
  symbols *incoming-stream* and *outgoing-stream*
* test/variables.lisp: removed; no longer used
* network.spread.asd (network.spread-test): removed file
  test/variables.lisp; switched to serial dependency structure
Support sequences instead of lists when specifying groups

* src/connection.lisp (join connection sequence): changed specializer
  list -> sequence
  (leave connection sequence): likewise
  (send-bytes connection sequence simple-array): likewise
  (send connection sequence simple-array): likewise; changed body
  accordingly
  (send connection sequence string): likewise
* src/ffi.lisp (%send-multiple): accept sequences of destinations
  instead of only lists
* test/connection.lisp (event-equal): new function; extracted from
  connection-root test suite
  (check-groups): likewise
  (drain-messages): likewise
  (check-membership-event): likewise
  (suite connection-root): removed local variables and functions
  (test connection-root::connect/smoke): renamed connect ->
  connect/smoke; added comments
  (test connection-root::membership/smoke): renamed membership ->
  membership/smoke; simplified group names; adapted to changed helper
  functions
  (test connection-root::membership-hooks): similar
  (test connection-root::send/smoke): renamed send -> send/smoke; use
  `ensure-cases'
TODO Added receive-into in src/connection.lisp

* src/protocol.lisp (header): updated copyright

  (receive-into): new generic function; receive data into an existing
  buffer

  (receive): added return-sender? and return-groups? keyword
  parameters; extended documentation string

* src/connection.lisp (receive-into connection simple-array): new
  method; receive into a given buffer

  (receive connection): allocate a buffer and use `receive-into';
  added keyword parameters return-sender? and return-groups? to
  control whether sender and/or groups should be extracted and
  returned

  (send connection string simple-array): change type check
  octet-vector -> simple-octet-vector
  (send connection sequence simple-array): likewise

* src/ffi.lisp (spread-receive): declared inline; prevent to/from
  string conversion of sender
  (with-pointers-to-vector-data): new macro; convenience wrapper for
  `cffi:with-pointer-to-vector-data'

  (%extract-type): renamed %type->tag -> %extract-type
  (%extract-sender): new function
  (%extract-groups):

  (%receive-into): renamed %receive -> %receive-into; receive into
  existing buffer; return type, sender and groups as multiple values;
  only return requested objects

  (%process-regular-message): removed; no longer used
  (%process-membership-message): likewise

* src/package.lisp (package network.spread): added used package
  nibbles; added exported symbol receive-into
* network.spread.asd (system network.spread): added system dependency
  on nibbles