Pattern matching

## Purpose
Refer https://stackoverflow.com/questions/2502354/what-is-pattern-matching-in-functional-languages

## Example
Shape = tags.case(circle.radius(Float)) case(square.side(Float))

```c
(this Shape).area = 
  this.
    case(circle.radius(0.0)):
      (0.0)
    case(square.side(0.0)):
      (0.0)
    case(circle.radius(r)):
      (pi.*(r.square))
    case(square.side(10.0)):
      (100.0)
    case(square):
      (99999)
```

## Warning
There are many kinds of pattern matching:

- For tagged union
- For Integer
- For String
- For Record types
- On function arguments!

## How to match function arguments?
Use record type matching as:

```
(this Boolean).or(that Boolean) =
  record.left(this) right(that).
    case(record.left(False) right(False)):
      (False)

    case(_):
      (True)
```

## Pattern matching with predicates
This can be achieve using lambdas. For example,
```
(this Shape).isBig = 
  case(circle.radius(.>(5.0)):
    (Boolean.true)
  case(square.side(.>(10.0)):
    (Boolean.true)
  case(rectangle.width(.>(5.0)) height(.>(5.0))):
    (Boolean.true)
  case(other):
    (Boolean.false)
```

Navigation Menu

Uh oh!

Pattern matching #40

Purpose

Example

Warning

How to match function arguments?