Inclusion is like importing, except that all variables that in scope
at the 'include' site are in scope of the included file.
Thus, if ./foo.nix contains 'x', then
let x = 1; in include ./foo.nix
evaluates to 1.
(Note that 'include' has to be a keyword, not a builtin function,
because inclusion takes place within a specific scope. I.e. 'include
./foo.nix' would not be equivalent to 'let f = include; in ... f
./foo.nix ...'.)
