Implement inexact, exact, exact->inexact and inexact->exact in scheme

[Katie Burke]

inexact should return an inexact representation of num
exact should return an exact representation of num
exact->inexact should return an inexact representation of num
inexact->exact should return an exact representation of num

( http://www.scheme.com/tspl4/objects.html#./objects:s112 )

This example should return 3.0

(inexact 3)

Traceback (most recent call last):
File "stdin", line 1, col 2
RunTimeError: unbound variable 'inexact'

This example should return 3

(exact 3.0)

Traceback (most recent call last):
File "stdin", line 1, col 2
RunTimeError: unbound variable 'exact'

This example should return 3.0

(exact->inexact 3.0)

Traceback (most recent call last):
File "stdin", line 1, col 2
RunTimeError: unbound variable 'exact->inexact'

This example should return 3

(inexact->exact 3.0)

Traceback (most recent call last):
File "stdin", line 1, col 2
RunTimeError: unbound variable 'inexact->exact'