Provide standard library as stdlib.y

[ydah]

doc: https://gist.github.com/ydah/fda6f4abac4ce34547fe90143eba33f2

[yui-knk]

I agree to the direction to define standard library in "stdlib.y" file, it makes clear for us what's each standard rule means.
However I have concerns for current design.

standard library by default or not

I think it's better to enable standard library by default. If some rules don't match with user's expectation, user can (or should be able to) overwrite the rule by themselves with writing same name parameterizing rule in their grammar file.

I aslo concern about the location where %include-stdlib can be written is very limited. As a user of parser generator, I don't want to take care of the location of each directive.

My suggestion is to make standard library by default.
In current design, grammar file parse and evaluation are separated. Parser#do_parse parsers grammar file and stores the structure into Grammar instance, Grammar#prepare is the method to instantiate rules. Both of them are called within Parser#parse but it's ok to move @grammar.prepare (and @grammar.validate!) outside Parser#parse.
We can merge the result of "stdlib.y" parse and grammar file parse before before instantiate rules.

The location of "stdlib.y"

"stdlib.y" needs to be included into gem package and https://github.com/ruby/ruby/tree/master/tool/lrama. We can manage both of them by configuring "lrama.gemspec" and changing install script. But I feel it's good to put "stdlib.y" under "lib", e.g. "lib/lrama/grammar/".

[ydah]

Thank you for the review. I agree that it should be enabled by default.
I have indeed made the modifications as I found that if I design it as you suggested, it will not be a problem.

The documentation has also been updated as per the current design.

[yui-knk]

Left some comments, others seems good to me!

[ydah]

Thank you for the review! I have updated this PR.

[ydah]

Thank you for the review! I have updated this PR.

[yui-knk]

Excellent job 💯