Fix cumulative and density function

[sunxd3]

• deviance function is not supported and user will be warned when using it.
• Compare to multibugs: multibugs doesn't seem to support cumulative and density, but provide individual cdf and pdf functions. We can also stop supporting these two functions, and direct user to use cdf and pdf.
• For now, a new member distributions is added to BUGSModel to track all the distributions: using Dict right now because non-scalar rv is problematic if we store distributions in the same way we store values. cumulative and density are handled in special way in _eval function to allow access to distribution info at run time. Maybe this can be generalized, the distribution can be helpful introspect info

Fix: #137

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 82.19%. Comparing base (fe710e6) to head (7a419e5).

Files	Patch %	Lines
src/compiler_pass.jl	91.17%	6 Missing ⚠️
src/utils.jl	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   81.78%   82.19%   +0.41%     
==========================================
  Files          21       21              
  Lines        1938     1966      +28     
==========================================
+ Hits         1585     1616      +31     
+ Misses        353      350       -3     

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[coveralls]

Pull Request Test Coverage Report for Build 8229663147

Details

• 129 of 145 (88.97%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.4%) to 82.224%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/utils.jl	40	42	95.24%
src/compiler_pass.jl	53	59	89.83%
src/parser/bugs_macro.jl	21	29	72.41%

Files with Coverage Reduction	New Missed Lines	%
src/utils.jl	1	85.78%

Totals
Change from base Build 8192126677:	0.4%
Covered Lines:	1619
Relevant Lines:	1969

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💛 - Coveralls

[sunxd3]

@yebai I removed some unrelated changes, the code change should be relatively clear.

I am merging this now, but if there are issue, I'll fix them later.