Add solutions to exercises from subsection 2.2.1

In exercise 2.17, there is an error with the test on hexlets site, which
should be reported to them.

ex-2.17.scm

@@ -0,0 +1,7 @@
+#lang sicp
+
+(define (last-pair list)
+  (cond ((null? list) (error "Can't get last element of empty list"))
+        ((null? (cdr list)) list)
+        (else (last-pair (cdr list)))))
+

ex-2.18.scm

@@ -0,0 +1,9 @@
+#lang sicp
+
+(define (reverse lst)
+  (define (rev-aux lst acc)
+    (if (null? lst)
+        acc
+        (rev-aux (cdr lst) (cons (car lst)
+                                 acc))))
+  (rev-aux lst '()))

ex-2.19.scm

@@ -0,0 +1,14 @@
+#lang sicp
+
+(define (first-denomination coin-values)
+  (car coin-values))
+(define (except-first-denomination coin-values)
+  (cdr coin-values))
+(define (no-more? coin-values)
+  (null? coin-values))
+
+; The answer will not be affected by the order of the list. We
+; are still dividing cases into "doesn't use the first coin"
+; and "uses at least one instance of first coin". However I
+; suspect it will impact the performace, although I haven't
+; formally proven it.

ex-2.20.scm

@@ -0,0 +1,10 @@
+#lang sicp
+
+(define (same-parity fst . lst)
+  (define (same-parity-aux parity lst)
+    (cond ((null? lst) '())
+          ((equal? parity (even? (car lst)))
+           (cons (car lst) (same-parity-aux parity (cdr lst))))
+          (else (same-parity-aux parity (cdr lst)))))
+  (same-parity-aux (even? fst)
+                   (cons fst lst)))

ex-2.21.scm

@@ -0,0 +1,11 @@
+#lang sicp
+
+(define (square x) (* x x))
+
+(define (square-list items)
+  (if (null? items)
+      nil
+      (cons (square (car items)) (square-list (cdr items)))))
+
+(define (square-list2 items)
+  (map square items))

ex-2.22.txt

@@ -0,0 +1,18 @@
+When we are running the iter line:
+(iter (cdr things)
+      (cons (square (car things))
+            answer))))
+We are essentially pushing on the "answer" accumulator list
+like a stack, so when we start with items = (1 2 3), answer = 
+().
+When we run the loop once we get:
+things = (2 3), answer = (1)
+things = (3), answer = (4 1)
+things = (), answer = (9 4 1).
+
+When Louis changes the order in the cons, he just produces the
+following:
+things = (1 2 3), answer = ()
+things = (2 3), answer = (() . 1)
+things = (3), answer = ((() . 1) . 4)
+things = (), answer = (((() . 1) . 4) . 9)

ex-2.23.scm

@@ -0,0 +1,7 @@
+#lang sicp
+
+(define (my-for-each func list)
+  (if (null? list)
+      #t
+      (begin (func (car list))
+             (my-for-each func (cdr list)))))