Added back `prob` and `logprob` macros

[torfjelde]

604 removed these two macros, but this will just result in ambiguous errors for lots of users (this feature has been displayed in the Turing.jl docs and we know there are people using it) without telling them anything about why the changes was made.

This PR makes them instead error, providing some information on where to find discussion + how to "replace" some of its functionality.

[coveralls]

Pull Request Test Coverage Report for Build 9126744062

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 77.596%

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Totals
Change from base Build 9126213178:	0.03%
Covered Lines:	2653
Relevant Lines:	3419

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💛 - Coveralls

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.27%. Comparing base (5b53be7) to head (6f15c32). Report is 3 commits behind head on master.

:exclamation: Current head 6f15c32 differs from pull request most recent head bedb345

Please upload reports for the commit bedb345 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #609 +/- ## ========================================== - Coverage 76.93% 76.27% -0.66% ========================================== Files 30 29 -1 Lines 3572 3439 -133 ========================================== - Hits 2748 2623 -125 + Misses 824 816 -8 ```

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