Turn the toy lisp into a complete R4RS implementation

[fitzgen]

why not.

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(checklist by @jorendorff)

Things we already sort of do, but sloppily:

• check we correctly parse identifiers
• check behavior of eqv? and equal?

Unsupported features:

• delay and force
• remaining caaaar functions not already covered in prelude.scm
• the numeric tower:
  • integers
    • big integer values, parsing, and write support
    • big integer support in generic builtins, like +
    • radix prefixes (#b, #o, #d, #x)
    • exactness prefixes (#i, #e)
    • gcd, lcm
  • flonums
    • floating-point number values, parsing, and write support
    • scientific notation (1.0e9)
    • exponent markers that indicate floating-point precision (s, f, d, l)
    • support in generic builtins, like +
    • exact? and inexact?, integer?
    • floor, ceiling, truncate, round, exp, log, sin, cos, tan, asin, acos, atan, sqrt, expt
  • exact rational numbers (3/4)
    • values, parsing, and write support
    • support in generic builtins
    • rational?
    • rationalize
  • complex numbers (3+4i, -i)
    • values, parsing, and write support
    • support in generic builtins
    • complex?, real?
    • make-rectangular, make-polar, real-part, imag-part, magnitude, angle
  • exact->inexact, inexact->exact
  • division /
  • string->number
• char->integer, integer->char
• Mutable strings: make-string, string-set! (ew), string-copy, string-fill!
• string-ci=? and other comparison procs
• substring
• with-input-from-file, with-output-to-file
• char-ready? (This is basically impossible to implement for us, so ... hmm. Always return true, I guess.)
• transcript-on, transcript-off