ir: add Parent field to Func and populate this field from Module.NewFunc

Updates #86.

ir/block.go

@@ -25,7 +25,7 @@ type Block struct {
 
 	// extra.
 
-	// Parent function; field set by ir.Function.NewBlock.
+	// Parent function; field set by ir.Func.NewBlock.
 	Parent *Func
 }
 

ir/func.go

@@ -67,6 +67,9 @@ type Func struct {
 	// (optional) Metadata.
 	Metadata
 
+	// Parent module; field set by ir.Module.NewFunc.
+	Parent *Module
+
 	// mu prevents races on AssignIDs.
 	mu sync.Mutex
 }

ir/func_block.go

@@ -2,6 +2,8 @@ package ir
 
 // NewBlock appends a new basic block to the function based on the given label
 // name. An empty label name indicates an unnamed basic block.
+//
+// The Parent field of the block is set to f.
 func (f *Func) NewBlock(name string) *Block {
 	block := NewBlock(name)
 	block.Parent = f

ir/module_func.go

@@ -6,8 +6,11 @@ import "github.com/llir/llvm/ir/types"
 
 // NewFunc appends a new function to the module based on the given function
 // name, return type and function parameters.
+//
+// The Parent field of the function is set to m.
 func (m *Module) NewFunc(name string, retType types.Type, params ...*Param) *Func {
 	f := NewFunc(name, retType, params...)
+	f.Parent = m
 	m.Funcs = append(m.Funcs, f)
 	return f
 }