feat[venom]: allow alphanumeric variables and source comments (#4403)

This commit implements minor improvements to the venom parser.
Specifically it allows for alphanumeric variable names, and for single
line source code comments in various styles: ";" "#" and "//"

---------

Co-authored-by: Charles Cooper <cooper.charles.m@gmail.com>

vyper/venom/parser.py

@@ -19,6 +19,9 @@
     %import common.WS
     %import common.INT
 
+    # Allow multiple comment styles
+    COMMENT: ";" /[^\\n]*/ | "//" /[^\\n]*/ | "#" /[^\\n]*/
+
     start: function* data_section?
 
     # TODO: consider making entry block implicit, e.g.
@@ -40,12 +43,10 @@
 
     CONST: INT
     OPCODE: CNAME
-    VAR_IDENT: "%" INT (":" INT)?
+    VAR_IDENT: "%" NAME
     LABEL: "@" NAME
     NAME: (DIGIT|LETTER|"_")+
 
-    COMMENT: ";"  /[^\\n]/*
-
     %ignore WS
     %ignore COMMENT
     """
@@ -59,7 +60,9 @@ def _set_last_var(fn: IRFunction):
                 continue
             value = inst.output.value
             assert value.startswith("%")
-            fn.last_variable = max(fn.last_variable, int(value[1:]))
+            varname = value[1:]
+            if varname.isdigit():
+                fn.last_variable = max(fn.last_variable, int(varname))
 
 
 def _set_last_label(ctx: IRContext):
@@ -172,7 +175,7 @@ def VAR_IDENT(self, var_ident) -> IRVariable:
         varname = parts[0]
         version = None
         if len(parts) > 1:
-            version = parts[1]
+            version = int(parts[1])
         return IRVariable(varname, version=version)
 
     def CONST(self, val) -> IRLiteral: