strings.textscanner, examples: add TextScanner .skip_whitespace/0, .peek_u8/0, .peek_n_u8/0, add mini_calculator_recursive_descent.v

[spytheman]

Here is a sample run of the new example, compared to bc:

#0 21:02:23 ^ feature/add/Textscanner.skip_whitespace ~/code/v>v run examples/mini_calculator_recursive_descent.v
Please enter the expression you want to calculate, e.g. `2 * (5-1)` .
Enter 'exit' or 'EXIT' to quit.
[1] 2 * (5-1)
8.0
[2] (((2+2)*3)+10)
22.0
[3] exit
#0 21:02:57 ^ feature/add/Textscanner.skip_whitespace ~/code/v>bc
2 * (5-1)
8
(((2+2)*3)+10)
22
#0 21:03:09 ^ feature/add/Textscanner.skip_whitespace ~/code/v>

_{Huly®: V_0.6-21442}

[spytheman]

Note: unlike examples/mini_calculator.v, examples/mini_calculator_recursive_descent.v can handle/skip whitespace, instead of erroring.

It is also shorter (since it reuses strings.textscanner for the trivial parts), and clearer to read/maintain (because of it using the recursive descent technique, instead of an explicit stack/array).