add examples

[CrockAgile]

Is your feature request related to a problem? Please describe.

rust has a convention of adding examples in the examples directory. and altho the documentation has some examples, it would be nice to add some examples where rust users expect to find them. and also may open the opportunity for more complex example.

Describe the solution you'd like

• one simple example of creating a Grammar, maybe even the same as the README
• one simple example of using a Grammar to parse a tree from input
• (stretch goal) one more complex example, using a parse tree to solve the simplest advent of code problem

Describe alternatives you've considered

• can the examples be included directly into the README ? not sure