String formatting ang string concatenation

[dnicolodi]

There would be value in adding support for rendering values as strings with some control on the formatting, and for concatenating strings.

Formatting could be done with a format() function resembling the homonymous Python function, or the Python str class method. The latter would allow to format multiple values in one string, with the drawback of a more complex implementation, especially regarding typing in BQL. The former would be much simpler to implement.

String concatenation could be done with the + operator like in Python or with the || like in standard SQL. I like the + better.