compiler: refactor and add tests

This commit finally introduces unit tests for the compiler, to check
whether input Go code is converted to the expected output IR.

To make this necessary, a few refactors were needed. Hopefully these
refactors (to compile a program package by package instead of all at
once) will eventually become standard, so that packages can all be
compiled separate from each other and be cached between compiles.

loader/ssa.go

@@ -16,3 +16,11 @@ func (p *Program) LoadSSA() *ssa.Program {
 
 	return prog
 }
+
+// LoadSSA constructs the SSA form of this package.
+//
+// The program must already be parsed and type-checked with the .Parse() method.
+func (p *Package) LoadSSA() *ssa.Package {
+	prog := ssa.NewProgram(p.program.fset, ssa.SanityCheckFunctions|ssa.BareInits|ssa.GlobalDebug)
+	return prog.CreatePackage(p.Pkg, p.Files, &p.info, true)
+}