Implement @offsetOf

[hvenev]

@offsetof behaves like @byteOffsetOf, with the exception that it fails
if the field does not begin at a byte boundary, i.e. the bit offset is
a multiple of 8.

[andrewrk]

For language changes, let's go through the proposal process for this first, please, then PR comes after it is accepted.

[hvenev]

What's the proposal process?

[mikdusan]

just create an issue for your proposal

[hvenev]

just create an issue for your proposal

Can I re-use #7794?

[SpexGuy]

Yes, but for a change like this (making offsetOf fail for bit packed fields), please add motivating examples of existing code where the change would be helpful to avoid bugs or footguns, or would make the code simpler.

[SpexGuy]

The process for language changes is:

1. A proposal issue is created proposing the idea.
2. Andrew will make a decision on it, either marking it as accepted or closing it. This sometimes takes a while, but if the issue makes a very good argument it can be fast.
3. PRs for accepted proposals may be merged.