error handling, fixed type mismatch.
	modified:   broom/repl.go
removed debug printing.
	modified:   broom/pair.go
added =, mod.
fixed sprintf, println bug, params are not evaled before use.
	modified:   broom/builtin.go
some tests
starting srfi-1 implementation with recur.
	modified:   broom/core.brm
	new file:   broom/core_test.brm
removed _debug
	modified:   broom/builtin.go
	modified:   broom/ea.go
	modified:   broom/sf.go
    go fmt
	modified:   broom/builtin.go
	modified:   broom/repl.go