Implement jump tables.

- Add a ir::jumptable module with a JumpTableData struct representing the vector
  of destinations.
- Add an entity map of jump tables to the Function.
- Parse and write jump tables in the function preamble.
- Rewrite EBB references in jumptables after parsing.

cranelift/tests/parser/branch.cton

@@ -43,3 +43,17 @@ ebb0(vx0: i32, vx1: f32):
 ebb1(vx2: i32, vx3: f32):
     brnz vx0, ebb0(vx2, vx3)
 }
+
+function jumptable() {
+    jt200 = jump_table 0, 0
+    jt2 = jump_table 0, 0, ebb10, ebb40, ebb20, ebb30
+
+ebb10:
+    trap
+ebb20:
+    trap
+ebb30:
+    trap
+ebb40:
+    trap
+}

cranelift/tests/parser/branch.cton.ref

@@ -37,3 +37,20 @@ ebb0(vx0: i32, vx1: f32):
 ebb1(vx2: i32, vx3: f32):
     brnz vx0, ebb0(vx2, vx3)
 }
+
+function jumptable() {
+    jt0 = jump_table 0
+    jt1 = jump_table 0, 0, ebb0, ebb3, ebb1, ebb2
+
+ebb0:
+    trap
+
+ebb1:
+    trap
+
+ebb2:
+    trap
+
+ebb3:
+    trap
+}