mbed.h imports namespaces

mbed.h should not import namespaces. This makes a decision about how to write programs for the developer instead of allowing them to make the decision. Changing this behaviour is a breaking change and will require a new major version.

https://github.com/ARMmbed/mbed-drivers/blob/master/mbed-drivers/mbed.h#L63
https://github.com/ARMmbed/mbed-drivers/blob/master/mbed-drivers/mbed.h#L64

In the mean time, users should be encouraged to include the API of only the portions of mbed drivers they are using, rather than all of mbed-drivers via the omni-include.

Read mbed-drivers/mbed.h at the two linked locations, lines 63-64, to inspect the namespace imports. Done means mbed.h no longer imports namespaces and the change is treated as the stated breaking change requiring a new major version; also review the omni-include guidance in the issue.

Written by the indexing model from the issue text.