[partial work] path: improve parse => format combination

make path.parse return an object where base is a computed property

Ref: https://github.com/nodejs/node/issues/1999

TODO:

• Add test
• Update doc with new capabilities

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

path