start work on aggregate types

The parser now has some code to parse the description
of structured types.

I tried to be even less specific than LLVM "type" by
creating just enough abstraction so that I can deal
with both AARCH64 and x64.  The base types used in the
definition of an aggregate are really just syntactic sugar
to give a structure a size and an alignment.  Only the
location of float components matters for the compilation.
In particular this means that the front-ends have to know
how the ABI works to pass/return small integer types.
This also means that the font-end has to be aware of the
layout of structures.

Chris Lattner has a proposition [1] for LLVM that is still
pending and goes in the direction of giving more control
of the ABI the front-end.

[1]: http://www.nondot.org/sabre/LLVMNotes/ExtendedIntegerResults.txt

lisc/main.c

@@ -24,7 +24,7 @@ main(int ac, char *av[])
 	int opt, pr;
 	Fn *fn;
 
-	fn = parsefn(stdin);
+	fn = parse(stdin);
 
 	pr = 1;
 	opt = 0;