Add a basic Pratt parser

[jahav]

The PR adds a basic Pratt parser that can parse numbers, arithmetic operations and numbers. In 3.0, it should replace current descent recursive parser.

What is a Pratt parser? A parser uniquely suited to parsing expressions. Instead of mucking around with Expressions, Factors, Terms, Atoms and all that rather ugly stuff in a recursive parser, Pratt parser is a modular and it's core is super simple (=maintainable). Since every formula in Excel is an expression, it's a match made in heaven. Here are some resources

• Demystifying Pratt Parsers - probably best explanation of the idea behind Pratt parser
• Pratt Parsers: Expression Parsing Made Easy - nice demonstration how to create a modular parser (which I shamelessly copy)
• Simple but Powerful Pratt Parsing - detailed, but idea gets kind lost in a code. Best to read after the other two.

Anyway, here are some other reasons why it's a great fit:

• Different formulas can't have some rules ([MS-XLSX] 2.2.2.1 -2.2.2.7). Thanks to modularity, I can easily create a parser that does that. Example:
  • cell formulas - no bang references
  • name formulas - can't use sheet-less cell references, can use macro function call
  • conditional format/data validation formulas - basically no reference operators allowed
• With new lexer design and more generic tokens, I don't have to have two special lexer for A1 and for R1C1. That goes back to bad design that had too much logic in lexer. I will be able to switch parselets from A1 to R1C1 and keep the rest more or less the same (I hope).