`bound_to_implemention` should affect implicit layer versioning

[ncoghlan]

The bound_to_implementation metadata flag indicates that the layer contents depend on the exact contents of the python_implementation field in the layer's base runtime definition.

Currently, changing the value of this field won't implicitly bump the version number of implicitly versioned layers, even if the affected layer is the relevant base runtime layer.

[ncoghlan]

Based on #230 it also looks like there is an implicit versioning gap around the deployment name of a layer's base runtime layer changing (as opposed to its build name)