S-Expressions

[tirix]

As suggested in a review comment in #65.

[digama0]

@tirix I made some changes directly on your branch:

• Changed s_expression to sexpr in method names
• Use a writer interface instead of generating and concatenating strings for fewer allocations
• Put a space between arguments so you get (a (b) (c)) instead of (a(b)(c))
• Print nullary applications as foo instead of (foo) so you get (a b c) instead of (a (b) (c)). (This should not be ambiguous since there is no way for ident s-exprs to get generated normally.)
• Use assert_eq instead of assert in tests. (This gives a more helpful error message in the not equal case, so it is most useful when the two sides have a nice debug print; since we're comparing strings that is the case here.)