Convert certain builtins into language constructs

[sagebind]

We gain very little by making everything into a builtin and we lose some degree of convenience. The following things should be made first-class language constructs:

• require: Define some sort of import syntax
• if
• cond
• while
• def: This currently uses hacks to be implemented as a builtin.
• set: This doesn't exist yet because it can't be implemented as a builtin.
• catch
• throw