Change the support code for directory creation so that it sets all
permission bits (ugo=rwx). The process's currently active umask will be
subtracted from these permissions, which leads to the result that (for a
standard umask of "022") directories are created with a mode of "0755".
So by default, directories created with `prim__createDir` are now also
group-executable and other-executable by default.
Previous behaviour was to create the directories readable for group and
other, but not executable (mode "744"), and thus inaccessible to anyone
except the owner.
Meaning that the FFI is aware of it, so you can send arbitrary byte data
to foreign calls. Fixes#209
This means that we no longer need the hacky way of reading and writing
binary data via scheme, so can have a more general interface for reading
and writing buffer data in files.
It will also enable more interesting high level interfaces to binary
data, with C calls being used where necessary.
Note that the Buffer primitive are unsafe! They always have been, of
course... so perhaps (later) they should have 'unsafe' as part of their
name and better high level safe interfaces on top.
This requires updating the scheme to support Buffer as an FFI primitive,
but shouldn't affect Idris2-boot which loads buffers its own way.
