Define model generator exactly as specified by the user

[devmotion]

This PR fixes #106 by defining a model generator with exactly the function signature that the user specified (i.e., all positional arguments and keyword arguments are preserved).

E.g.,

@model function test1(x, y = 4.0)
    x ~ Normal(0, 1)
    y ~ Exponential()
end

defines the model generator test1(x, y = 4.0), whereas

@model function test2(x; y = 4.0)
    x ~ Normal(0, 1)
    y ~ Exponential()
end

defines the model generator test2(x; y = 4.0). Thus in the first case one can generate a model with test1(3.0) and test1(3.0, 10.0) but not test1(3.0, y = 10.0), whereas in the second case one can define a model with test2(3.0) and test2(3.0; y = 10.0) but not test2(3.0, 10.0).

[devmotion]

I also played around a bit with bors in my own repos, and it seems the status has to be specified differently for Github actions.

[devmotion]

bors try

[codecov]

Codecov Report

Merging #109 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   74.91%   74.97%   +0.05%     
==========================================
  Files          14       14              
  Lines         901      903       +2     
==========================================
+ Hits          675      677       +2     
  Misses        226      226              

Impacted Files	Coverage Δ
src/compiler.jl	88.03% <100.00%> (+0.20%)	⬆️

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[bors]

try

Build succeeded:

• test (1, ubuntu-latest, x64)
• test (1.0, ubuntu-latest, x64)
• test (1, ubuntu-latest, x86)
• test (1, windows-latest, x64)
• test (1, macOS-latest, x64)
• test (1.0, macOS-latest, x64)
• test (1.0, ubuntu-latest, x86)
• test (1.0, windows-latest, x64)
• test (1.0, windows-latest, x86)
• test (1, windows-latest, x86)
• continuous-integration/travis-ci/push