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/layout.rs

@@ -6,6 +6,8 @@
 use std::iter::{Iterator, IntoIterator};
 use entity_map::{EntityMap, EntityRef};
 use ir::entities::{Ebb, NO_EBB, Inst, NO_INST};
+use ir::dfg::DataFlowGraph;
+use ir::builder::Builder;
 
 /// The `Layout` struct determines the layout of EBBs and instructions in a function. It does not
 /// contain definitions of instructions or EBBs, but depends on `Inst` and `Ebb` entity references
@@ -615,6 +617,11 @@ impl<'f> Cursor<'f> {
         }
     }
 
+    /// Create a builder for inserting an instruction at the current position.
+    pub fn ins<'c, 'fd>(&'c mut self, dfg: &'fd mut DataFlowGraph) -> Builder<'c, 'f, 'fd> {
+        Builder::new(dfg, self)
+    }
+
     /// Insert an EBB at the current position and switch to it.
     ///
     /// As far as possible, this method behaves as if the EBB header were an instruction inserted

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;