from/to string conversion for primitive expressions

[julochrobak]

Bandicoot is a strongly-typed language and converting a primitive expression of a given type into another type must be explicit. The current version of Bandicoot supports only conversion from one numeric type to another. This task is to implement conversions from string to numeric and the other way around:

extend si = (string 1), sr = (string 1.1), sl = (string 1L)
i = (int "123234" 0), r = (real "123123.3242343" 0.0), l = (long "12312312321" 0L)
....

The conversion from a string to a numeric value must have a default value specified. The default is used if the conversion fails:
i = (int "Hello World" 0)