Relationship between tags and record field names

[aherrmann]

closes #8

Following the direction of #28

• Modifies Field and Pos such that the resulting tag matches the field name or position.
• Adds a Rename combinator that allows to rename the tag e.g. from position 0 to tag "foo".
• Updates the examples accordingly

[aspiwack]

It reads a bit weird that Rename mentions the old tag but not the new tag. But we can sort this out later.

[aherrmann]

It reads a bit weird that Rename mentions the old tag but not the new tag. But we can sort this out later.

That's true. We could add the new tag as a parameter to Rename, but it would technically be redundant. The same is true for the field/pos parameter on Field/Pos. Currently, it's a redundant parameter. We should probably be consistent and either always or never have such technically redundant parameters.