Use the term "Function parameter" instead of "argument".

Rename the ArgumentType type to AbiParam since it describes the ABI
characteristics of a parameter or return value, not just the value type.

In Signature, rename members argument_types and return_types to "params"
and "returns". Again, they are not just types.

Fix a couple lingering references to "EBB arguments".

lib/cretonne/src/ir/mod.rs

@@ -24,7 +24,7 @@ mod valueloc;
 pub use ir::builder::{InstBuilder, InstBuilderBase, InstInserterBase, InsertBuilder};
 pub use ir::dfg::{DataFlowGraph, ValueDef};
 pub use ir::entities::{Ebb, Inst, Value, StackSlot, GlobalVar, JumpTable, FuncRef, SigRef, Heap};
-pub use ir::extfunc::{Signature, CallConv, ArgumentType, ArgumentExtension, ArgumentPurpose,
+pub use ir::extfunc::{Signature, CallConv, AbiParam, ArgumentExtension, ArgumentPurpose,
                       ExtFuncData};
 pub use ir::funcname::FunctionName;
 pub use ir::function::Function;