Add macro literals

[hrs]

We've got function literals in the form of lambdas, and macros are essentially a first-class object in Blueprint anyway, so maybe we should add a syntax to create anonymous macros?

[hrs]

As mentioned in #39, our macros are really fexrs. What syntax should we use? Maybe macro or fexpr instead of lambda?

[hrs]

Definitely fexpr, since we renamed macro → fexpr in c3e7a8a.

A new question: suppose we implement fexpr literals and use fexpr to create them. Should we use fun instead of lambda to create function literals to match the defun/deffexpr symmetry?