Add a Cursor::ins() method which constructs a Builder.

Rewrite Builder uses in test cases to use this method and construct a
new builder for each instruction. This pattern allows us to change the
InstBuilder trait to a one-shot implementation that can only create a
single instruction.

Don't re-export the Builder struct, it is less important than the
InstBuilder trait, and we may get more implementations.

lib/cretonne/src/ir/mod.rs

@@ -24,4 +24,4 @@ pub use ir::jumptable::JumpTableData;
 pub use ir::dfg::{DataFlowGraph, ValueDef};
 pub use ir::layout::{Layout, Cursor};
 pub use ir::function::Function;
-pub use ir::builder::{Builder, InstBuilder};
+pub use ir::builder::InstBuilder;