Add signatures and ext_funcs tables to DataFlowGraph.

These two tables are used to keep track of type signatures of function
calls as well as external function references used in direct function
calls.

Also add an ExtFuncData struct representing an external function that
can be called directly.

lib/cretonne/src/ir/mod.rs

@@ -15,7 +15,7 @@ mod extfunc;
 mod builder;
 
 pub use ir::funcname::FunctionName;
-pub use ir::extfunc::{Signature, ArgumentType, ArgumentExtension};
+pub use ir::extfunc::{Signature, ArgumentType, ArgumentExtension, ExtFuncData};
 pub use ir::types::Type;
 pub use ir::entities::{Ebb, Inst, Value, StackSlot, JumpTable, FuncRef, SigRef};
 pub use ir::instructions::{Opcode, InstructionData, VariableArgs};