Speedup: PrettyPrint, Render, and Parse (#12)

- Use string builder for Render and PrettyPrint
- Use runes instead of strings for current char comparison
- Use sets instead of arrays
- Build the regex exactly once
- Use token as a rune array
- Give size and cap for `fTokens`
- Add test for non-nil tokens and fix issue
- Add 3 formula function errors
- Add comments for the helper function
- Define helper functions after constants, variables, and data types
- Simplify data type declaration in the function parameters
- Remove outdated URL links in the documentation

PULL_REQUEST_TEMPLATE.md

@@ -21,7 +21,7 @@
 
 <!--- Please describe in detail how you tested your changes. -->
 <!--- Include details of your testing environment, and the tests you ran to -->
-<!--- see how your change affects other areas of the code, etc. -->
+<!--- See how your change affects other areas of the code, etc. -->
 
 ## Types of changes
 

README.md

@@ -49,7 +49,7 @@ Contributions are welcome! Open a pull request to fix a bug, or open an issue to
 
 ## Credits
 
-EFP (Excel Formula Parser) is a Golang port of [E. W. Bachtal's](https://ewbi.blogs.com/develops/2004/12/excel_formula_p.html) Excel formula parser.
+EFP (Excel Formula Parser) is a Go language port of E. W. Bachtal's Excel formula parser.
 
 ## Licenses