implement support for unversioned pins

[machow]

Background

For full compatibilty with R pins, a user should be able to do the following:

• write for the first time an unversioned pin
• follow a process to switch a versioned pin to an unversioned one
  • e.g. receive an error, with instructions when multiple versions detected.
• set versioning defaults as a board level option
• specify a pin should be unversioned in pin_write

Relevant issues:

• Research the path that board$versioned takes in R pins #16

Research needed

• Do we want to deviate from any of these behaviors?

[machow]

Note that R pins has pins_inform messages for creating / replacing of versions

[isabelizimm]

There are now read-only unversioned pins with #213