Fix circular file dependency

[Gleefre]

Fixes #36 by splitting constant definitions into two files.

Specifically, infinity constants are moved to inifinity.lisp to be loaded before float-features.lisp, and nan constants are moved to nan.lisp to be loaded after (as before the merge in da01c1c).

This is needed because the infinity constants are used in the definition of bits-short-float, which in turn is used to define SHORT-FLOAT-NAN:

float-features/float-features.lisp

Lines 353 to 356 in e1202d9

	((zerop sign)
	single-float-positive-infinity)
	(t
	single-float-negative-infinity))

float-features/constants.lisp

Lines 109 to 116 in e1202d9

	(handler-case
	(progn
	(bits-short-float 0)
	(defconstant SHORT-FLOAT-NAN
	(bits-short-float #b0111111000000000)))
	(error ()
	(define-symbol-macro SHORT-FLOAT-NAN
	(bits-short-float #b0111111000000000))))

[Shinmera]

Thanks!