Allow binding of reserved words in LET
Reimplement IF as a macro and binary conditional special form
Don't support non-nil atom arguments to JOIN
Add bindings to the appropriate module directly in SETENV
Remove unneccessary check in PROLOGUE
Minor tweaks to EXPORTED
Don't wrap the first two arguments to %FOR in a list

This makes it more consistent with other special forms and removes a
special case from MACROEXPAND when the arguments to %FOR could look
like a macro call.
Minor fix in ENCAPSULATE: BODY is a list of forms
Work in progress on full lowering pass