[WIP] Added InitialMapper base class and LineInitialMapper implementation

[ammareltigani]

This PR is the second in a series to implement qubit routing capabilities in Cirq. It (1) adds a base class AbstractInitialMapper (2) an implementation of this base class LineInitialMapper that uses the placement strategy outlines in https://arxiv.org/pdf/1902.08091.pdf

For more context, see http://tinyurl.com/cirq-qubit-routing