Syntax: support multiprecision integer literals (#58)

* Syntax: support multiprecision integer literals

* Using INT token type for both cases
the parser is handling int64 and *big.Int values as a token of the type INT

* Tests for bigInt

* Testing large integer literals

* Deleting some comments

* Fixing a space
diff --git a/syntax/parse.go b/syntax/parse.go
index e4c5644..45bc7ab 100644
--- a/syntax/parse.go
+++ b/syntax/parse.go
@@ -778,7 +778,11 @@
 		tok := p.tok
 		switch tok {
 		case INT:
-			val = p.tokval.int
+			if p.tokval.bigInt != nil {
+				val = p.tokval.bigInt
+			} else {
+				val = p.tokval.int
+			}
 		case FLOAT:
 			val = p.tokval.float
 		case STRING: