Expose nmb_expect to Objective-C

- Move expect DSL functions into Objective-C-bridged DSL class.
- Define macro nmb_expect, which calls new DSL class.
- Bridge Prediction to Objective-C in order to allow
  `nmb_expect(...).to` syntax. This causes matcher extensions to define
  duplicate methods in Objective-C [1]. Mitigate this error by using
  "nmb_"-prefixed names in Objective-C.
- Add defines that remove "nmb_" prefix for beNil, beTrue, and beFalse
  matchers.

[1] https://devforums.apple.com/thread/229668?tstart=0)
Expose nmb_expectBlock to Objective-C
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