exercise 1.24, README.md, and imports for large numbers

1. In order to use fermat-test with large numbers, it was necessary
to add the numbers egg because for very large numbers, scheme reverts
to imprecise flownums.

2. The (random n) function has an upper limit limitation, based on
GNU C's rand() function, which has an upper-limit of RAND_MAX, which
has an upper limit of 32,767 on some machines and 2^31-1 otherwise.

3. modified the README.md file to reflect the fact that the chapter
sections are now divided into separate files.

4. solved exercise 1.24