Ch. 1 exercises through 1.2.5

Signed-off-by: Christian Romney <cromney@pointslope.com>
Changed implementation of subst to follow book

This version follows the grammar and uses one function
for each of sexp and slist

Signed-off-by: Christian Romney <cromney@pointslope.com>
Inlined subst-in-sexp as per exercise 1.12

Signed-off-by: Christian Romney <cromney@pointslope.com>
Created subst-map for exercise 1.13

Signed-off-by: Christian Romney <cromney@pointslope.com>
Inlined letrec into anonymous lambda in subst-map

Signed-off-by: Christian Romney <cromney@pointslope.com>