mmap for guile

[Matt Wette <matt.wette@gmail.com>]

I have a candidate mmap implementation for guile (3.0.8) in

	github.com/mwette/guile-contrib/patch/3.0.8/mmap-api.patch

It probably needs some review and update.  Maybe windows mmap-file would be nice.
I'm not sure I updated the configure correctly but --disable-mmap-api should remove.
It exposes features 'mmap-api and 'mmap-file.

scheme@(guile-user)> ,d mmap
- Scheme Procedure: mmap addr len [prot [flags [fd [offset]]]]
      See the man page.  Returns a bytevector.  Note that the region
      returned by mmap will NOT be searched by the garbage collector for
      pointers.  See also mmap/search.  Defaults are:

      prot
           (logior PROT_READ PROT_WRITE)

      flags
           (logior MAP_ANON MAP_PRIVATE)

      fd
           -1

      offset
           0

scheme@(guile-user)> ,d mmap/search
- Scheme Procedure: mmap/search addr len [prot [flags [fd [offset]]]]
      See the unix man page for mmap.  Returns a bytevector.  Note that
      the region allocated will be searched by the garbage collector for
      pointers.  Defaults:

      prot
           (logior PROT_READ PROT_WRITE)

      flags
           (logior MAP_ANON MAP_PRIVATE)

      fd
           -1

      offset
           0

scheme@(guile-user)> ,d mmap-file
- Scheme Procedure: mmap-file file [prot]
      This procedure accepts a file in the form of filename, file-port or
      fd.  It returns a bytevector.  It must not contain scheme allocated
      objects as it will not be searched for pointers.  Default PROT is
      `"r"'.