feat!: simplify SignatureFunc and add custom arg validation. (#706)

Closes #675 but not exactly as specified in the issue - SignatureFunc is
still an enum, but something close to the desired interface is achieved.

BREAKING_CHANGES: `Extension::add_op...` methods simplified and renamed,
there is only `add_op` and `add_op_simple` now.

src/extension.rs

@@ -22,7 +22,10 @@ mod infer;
 pub use infer::{infer_extensions, ExtensionSolution, InferExtensionError};
 
 mod op_def;
-pub use op_def::{CustomSignatureFunc, OpDef};
+pub use op_def::{
+    CustomSignatureFunc, CustomValidator, OpDef, SignatureFromArgs, ValidateJustArgs,
+    ValidateTypeArgs,
+};
 mod type_def;
 pub use type_def::{TypeDef, TypeDefBound};
 pub mod prelude;