Add improved input facilities

[eholk]

Aaron Clarke was asking about how to get input in Harlan programs. Currently there's not a lot of support for this, although you might be able to get some done by using extern to call C functions. Still, a better API would be better. Aaron would like to be able to do something like this:

(let ((temperatures (load-vector "temp.dat")))...)

or

(let ((features (load-matrix (argv 1))))...)

[eholk]

1f18a0d3e8 includes some more useful input functions, although still not quite as high level as suggested by Aaron.