Refactor mitscheme/book.mit, pulling common code into common/book.scm.

This is the first pass of the refactoring. Eventually,
mitscheme/toplevel.mit will go away, and that code will be moved into
common/book.scm as well.
Move START and TEST into common/book.scm.

Re-organize the definitions in mitscheme/mit-book.mit to make porting to
other scheme's easier.
Don't eval define-generic forms for show and clone.
Remove unused error handling definitions.
Move error handling code closer together.
Changes to allow Gambit scheme to use common/book.scm.

- Wrap show and clone in an eval to delay macro-expansion of those
  forms. Gambit apparently only attempts macro-expansion once before
  other files are loaded, so we need to delay the expansion of
  define-generic until after meroonet.scm is loaded. Another option
  would be to put forms that use macros defined by meroonet in a
  separate file and load that file as well.

- Make define-abbreviation expand directly into a define-macro form,
  rather than into a define-meroonet-macro form which itself expands
  into define-macro. This prevents the need to wrap
  define-meroonet-macro in an eval for similar reasons to those
  mentioned above.

- The The gambit files no longer use include, so there's no need to
  rewrite the include paths to absolute paths in the Makefile.
Move code from rtbook+.bgl into rtbook.bgl.

According to the source comments, these were originally split into two
files to reduce compilation times.

This is a step in the direction of using common/book.scm in bigloo.
Split common/book.scm into smaller modules.

In order to accommodate bigloo's module system, split the monolithic
common/book.scm into smaller modules, so bigloo/rtbook.bgl can pick and
choose which ones it wants to include.
Split common/definitions.scm into several files.

Split some definitions out into separate files under common/compat.
Functions in common/compat are either not provided by some schemes, or
else an incompatible version is provided. Break these into separate
files to allow each interpreter/book.scm to selectively include the ones
it needs.

Also, switch bigloo over to using the common code for definitions.scm,
generics.scm and toplevel.scm.
Remove TODO about moving common code out of <interpreter>/book.scm.