Major syntax overhaul

- Object/field names/enum keys are now normalized to typical Scheme
  style (lowercase, dashed)

- Valueparams are now provided as a keyword argument list; it isn't
  necessary to specify which enum is used anymore

- Strings can be passed directly to xcb requests and will be converted
  to the corresponding X protocol data types transparently

- Strings can be read out of X data structures using the xref-string
  procedure

- Values can be passed as valueparam args without first converting
  them to integers

- XCB request procedures do not take an xcb connection as an argument;
  rather, they use (current-xcb-connection), which has been moved to
  the core library

- New version of each request procedure with "/c" appended to its name
  that does take an xcb connection as before

- Test cases updated

Other changes

- Fix big in event loop that caused newly reified continuations to
  store the "parent" continuation in their closure, which was using up
  all the memory

- Extension enabling procedures no longer require the event loop to
  run; a version that takes advantage of the event loop has been added
  to the event loop code