SUBST (#320)

* WIP: Yarn Build works

* WORKING: Add treeifier

* Working prototype: Source 1 substituter

* Fix blockExpression in source 1, started source 2

* Fix block expression and logical expression

1. Now logical expressions are evaluated into expressions
2. Block expressions with a single statement as body, and if the
    statement is an Expression Statement with irreducible expression,
    the block will be reduced into undefined.

* Bump version (#325)

* Added builtin functions for source 1, 2.

* Add tests for substituter

* Add ArrayExpression as an irreducibleNode

Fix stringify.

* Add constants prelude

Fix also the complications of Infinity and NaN

* Remove debugging statements and update tests

* Fix problems with builtin functions

* Add tests

* Add substituter and treeifier for LogicalExpr

and update test snapshots

* Fix return values of builtin math functions

with refactoring

* Fix builtin math on neg argument and is_function

* Function equality uses javascript's ===

update tests

* Remove debugging statements and update test snapshots

* 0.3.0

* Eliminate substituter hangs from using libraries

* 0.3.1

package.json

@@ -1,6 +1,6 @@
 {
   "name": "js-slang",
-  "version": "0.2.6",
+  "version": "0.3.1",
   "description": "Javascript-based interpreter for slang, written in Typescript",
   "author": {
     "name": "Source Academy",
@@ -14,7 +14,7 @@
     "source-map": "^0.7.3"
   },
   "main": "dist/index",
-    "types": "dist/index",
+  "types": "dist/index",
   "files": [
     "dist"
   ],