Implemented a basic API for SSA

src/ssa/Block.go

@@ -1,11 +1,12 @@
 package ssa
 
+// Block is a list of instructions that can be targeted in branches.
 type Block struct {
-	Entry        []*Block
 	Instructions []Instruction
-	Exit         []*Block
 }
 
-func (b *Block) Append(instr Instruction) {
+// Append adds a new instruction to the block.
+func (b *Block) Append(instr Instruction) *Instruction {
 	b.Instructions = append(b.Instructions, instr)
+	return &b.Instructions[len(b.Instructions)-1]
 }