Add FileSystem abstraction with native posix implementation

A new basis.io package contains the FileSystem abstraction. And the
posix implementation exists in the new basis.platform package. The
platform package has two goals: to provide minimally abstracted access
to each operating system's API, and to make easy things easy.
Commonality among platform APIs will bubble up to the abstraction layer
when appropriate.

Modelling memory mapped files as coherent, shared-memory Data objects
serves as the initial motiviation for this subsystem. Ideally,
basis.platform will encapsulate all native code.

Note that JNI library loading has not been implemented yet. By default,
I plan to copy an embedded dynamic library to a temporary file from
which it will be loaded. Building and publishing cross-platform jars
will be challenging to automate.