Generate a read proc with a static reg arg

[dwhall]

The static register argument to the read() proc allows Nim to optimize
the code and the result is faster execution. To use this feature,
the calling Nim source code must be of the form:

var regval = PERIPHERAL.REGISTER.read()

or

const reg = PERIPHERAL.REGISTER
var regval = reg.read()

The following calling conventions will use the legacy read() that will not be optimized:

let reg = PERIPHERAL.REGISTER
var regval = reg.read()

var reg = PERIPHERAL.REGISTER
var regval = reg.read()

[dwhall]

Read more about the details of this change here. I've found since that was written that applying static to the write() proc is not helpful.

If this PR's change works and doesn't hurt the consumers of this project, we should also consider applying a similar change to modifyIt()