compiler: refactor interface invoke wrapper creation

Now that most of the utility compiler methods are ported over to the
builder or compilerContext, it is possible to avoid having to do the
wrapper creation in two steps. A new builder is created just to create
the wrapper.

This is a small reduction in line count (and a significant reduction in
complexity!), even though more documentation was added.

compiler/llvm.go

@@ -64,6 +64,13 @@ func (c *Compiler) emitPointerPack(values []llvm.Value) llvm.Value {
 	return llvmutil.EmitPointerPack(c.builder, c.mod, c.Config, values)
 }
 
+// emitPointerPack packs the list of values into a single pointer value using
+// bitcasts, or else allocates a value on the heap if it cannot be packed in the
+// pointer value directly. It returns the pointer with the packed data.
+func (b *builder) emitPointerPack(values []llvm.Value) llvm.Value {
+	return llvmutil.EmitPointerPack(b.Builder, b.mod, b.Config, values)
+}
+
 // emitPointerUnpack extracts a list of values packed using emitPointerPack.
 func (c *Compiler) emitPointerUnpack(ptr llvm.Value, valueTypes []llvm.Type) []llvm.Value {
 	return llvmutil.EmitPointerUnpack(c.builder, c.mod, ptr, valueTypes)