Adding environment: option to repl.
Fixes issue #180.
Clarifying PREFIX should specified to make in addition to make install.
Wrapping continuable exceptions in specially tagged exceptions rather than pairs.
This way C functions that check for and print exceptions support them
automatically.  Notably continuable errors in macro expansions, will had
temporarily been suppressed, are printed again.
Refactoring repl.
The guard-like macro in the core language is now called protect.
This is the recommended syntax for error-handling in Chibi-specific
programs, since it's not possible to get stack traces when using
R[67]RS guard.  guard is defined separately and according to the
standard in (scheme base).