support optional padding specifiers

This adds support for (optional) padding specifiers on the parameters of
assembly functions.  This allows one to effectively provide a "witness"
which is always used within the padding region.

Author: DavePearce

pkg/asm/assembler/parser.go

@@ -170,9 +170,11 @@ func (p *Parser[F]) parseArgsList(kind schema.RegisterType) ([]io.Register, []so
 				return nil, errs
 			}
 		}
-		// parse name & type
+		// parse name, type & optional padding
 		if arg, errs = p.parseIdentifier(); len(errs) > 0 {
 			return nil, errs
+		} else if padding, errs = p.parseOptionalPadding(); len(errs) > 0 {
+			return nil, errs
 		} else if width, errs = p.parseType(); len(errs) > 0 {
 			return nil, errs
 		}
@@ -185,6 +187,23 @@ func (p *Parser[F]) parseArgsList(kind schema.RegisterType) ([]io.Register, []so
 	return regs, nil
 }
 
+func (p *Parser[F]) parseOptionalPadding() (big.Int, []source.SyntaxError) {
+	var (
+		padding   big.Int
+		errs      []source.SyntaxError
+		lookahead lex.Token
+	)
+	//
+	if !p.match(EQUALS) {
+		// no optional padding
+		return padding, nil
+	} else if lookahead, errs = p.expect(NUMBER); len(errs) > 0 {
+		return padding, errs
+	}
+	//
+	return p.number(lookahead), nil
+}
+
 func (p *Parser[F]) parseType() (uint, []source.SyntaxError) {
 	var (
 		lookahead = p.lookahead()

testdata/asm/add.zkasm

@@ -6,7 +6,7 @@
 ;; SUB    (ARG_1 - ARG_2) % 2^256                   0x03
 ;;
 ;; Both of these operate on u256 words with "wrap around" semantics.
-fn add(INST u8, ARG_1 u256, ARG_2 u256) -> (RES u256) {
+fn add(INST=1 u8, ARG_1 u256, ARG_2 u256) -> (RES u256) {
   var c u1
   ;;
   if INST == 0x01 goto insn_add

testdata/asm/wcp.zkasm

@@ -24,7 +24,7 @@
 ;; NOTE: an undocumented feature of the WCP module is that it is often
 ;; used by other modules to additionally ensure that the given
 ;; arguments are "small" (i.e. within their designated bitwidths).
-fn wcp(INST u8, ARGUMENT_1 u256, ARGUMENT_2 u256) -> (RESULT u1) {
+fn wcp(INST=1 u8, ARGUMENT_1 u256, ARGUMENT_2 u256) -> (RESULT u1) {
    var delta u256
    var arg1bits, arg2bits u255
    var borrow,arg1sign,arg2sign u1