Add document path equivalent to path to ResolveInfo

[nickbruun]

This extends the behavior introduced in #148 by adding a document path to ResolveInfo, ie. the path as the final response will see it, such that it is possible to refer to a path equivalent to the aliased path. This serves to make it possible to use the path to describe error paths in custom error messages.

Further, this copies the ResolveInfo with the updated paths for list element iteration, to avoid mutating the ResolveInfo of the field as well as to prevent race issues between list elements accessing the path in promises.

[nickbruun]

On second thought, there is another way in which we can track the path that makes this a little less use-case specific. I don't know if this is a change that's acceptable given that the path addition has already been in master for a while, but essentially, we could describe the path as a list of AST nodes and list indices, rather than string and list indices. This way, an end user could easily evaluate the path.

[syrusakbary]

Hi @nickbruun,

Thanks to your PR I realized there was a small bug in the ResolveInfo path for aliased fields.
I've fixed it with this commit: graphql-python/graphql-core@0b3cb44

@ekampf

I'm closing this PR since your changes shouldn't be necessary now.
Feel free to reopen it (or comment) if you see something buggy :)

[nickbruun]

This does the trick perfectly :) Thanks!