device builder API

[tanriol]

Split Context into Builder (device not opened yet, can choose interface) and Device (interface chosen, device opened, can interact with it). This makes a number of API misuses, such as trying to change interface on the fly or send data before opening interface, impossible.

[tanriol]

@cr1901 I'm pretty sure I'll merge this for the next version, possibly with some changes (for example, I can re-introduce Context as an internal wrapper over the *mut ftdi_context), so you may want to check whether and how it affects error handling.

[cr1901]

@tanriol Merge it, I'll handle the fallout of merging it back into my branch (merge commit is okay? It's preferable to a rebase for me at this point.).

(for example, I can re-introduce Context as an internal wrapper over the *mut ftdi_context), so you may want to check whether and how it affects error handling.

What would the different between Context and Device be then? Context provides lower-level methods that directly call into libftdi1 and Device provides higher-level methods?

[tanriol]

What would the different between Context and Device be then? Context provides lower-level methods that directly call into libftdi1 and Device provides higher-level methods?

I decided to go without that for now, but it would have been Context providing the Drop impl and functionality shared between Device and Builder (if any). However, it looks like there's not much common functionality there.