Remove magic number, do clean work on ex3.4
Add inexact random number support in ex3.5
Change global variable to local variable binding in ex3.8
Add SICP execise 3.18
Add SICP exercise 3.19
Add SICP exercise 3.21
Add SICP exercise 3.22
Add SICP exercise 3.23
Add SICP exercise 3.24
Add SICP exercise 3.25