Add a source generator for a Maths language to the samples

[lucabol]

The scenario is that a financial/gas/whatever-scientific company wants their mathematicians to write their formulas with a syntax familiar to them and without the risk of introducing bugs. But they also need to run as fast as compiled code (i.e. inline), because that is what their business relies on.

Enter source generators. They now can write:

AreaSquare(l)       = pow(l, 2)
AreaRectangle(w, h) = w * h
AreaCircle(r)       = pi * r * r
Quadratic(a, b, c)  = {-b + sqrt(pow(b,2) - 4 * a * c)} / (2 * a)

PointDist(x', x'', y', y'')     = sqrt(pow([x'-x''],2) + pow([y'-y''], 2))

I needed to build a lexer / parser for the language instead of simple regex substitution to avoid the possibility of an injection attack by a disgruntled employee. I built it in the simplest possible way for the sake of example.

The language supports:

1. Using prime char in identifiers
2. Using differents parenthesis
3. Calling functions in the Math class lowercase and functions defined before in the file (no other functions allowed)
4. Simpler syntax than C# code (i.e. no types etc...)

This is not completed yet, but done a pull request for the sake of review.

[jmarolf]

@lucabol can you pull in the latest changes from master? SourceGeneratorContext no longer exists in the public API and all of our samples where updated here: #625

[lucabol]

I am closing this PR as it was created from an old fork. #695 represents the new one we are merging.