[MLton] basis library -> library project

[Stephen Weeks]

One "broken link" with the versioning approach with the new library
project is the basis library.  It can change over time, yet the
semantics of those changes would not be reflected by version changes
in library paths.  Those changes could then change the types/behavior
of libraries depending on the basis, and those changes also would not
be reflected by version changes in library paths.

What do people think about moving the basis library to the library
project, at org/mlton/basis/, to fix this problem?


At the next level down, the problem still exists with changes to
compiler primitives.  But since the basis library is so intertwined
with those, we should be able to arrange to bump the version when
those change.