…inition (#237)

The motivation for the PR is initially to allow the use
[`product_distribution`](https://juliastats.org/Distributions.jl/stable/multivariate/#Distributions.product_distribution)
in JuliaBUGS model. By writing

```julia
@bugs begin
    x[1:2] ~ Distributions.product_distribution(fill(Normal(), 2))
end
```

It also enables

```julia
julia> foo(x) = x + 1
foo (generic function with 1 method)

julia> model_def = @bugs begin
           a = Main.foo(b)
       end
quote
    a = Main.foo(b)
end

julia> compile(model_def, (;b=2))
BUGSModel (parameters are in transformed (unconstrained) space, with dimension 0):

  Model parameters:

  Variable sizes and types:
    a: type = Int64
    b: type = Int64
```

i.e., an easier way to introduce external functions into JuliaBUGS. 

This can't fully replace `@register_primitive` yet, because using `Main`
module is relying on Julia runtime behavior and not very intuitive.
Examples:

```julia
julia> module TestModule
           bar(x) = x + 1
       end
Main.TestModule

julia> model_def = @bugs begin
           a = TestModule.bar(b)
       end
quote
    a = TestModule.bar(b)
end

julia> compile(model_def, (; b = 1))
ERROR: UndefVarError: `TestModule` not defined in `JuliaBUGS`
...
```

one would need to do 

```julia
julia> model_def = @bugs begin
           a = Main.TestModule.bar(b)
       end
quote
    a = Main.TestModule.bar(b)
end
```

also

```julia
julia> @testset "t" begin
           foo1(x) = x + 1
           model_def = @bugs begin
               a = Main.foo1(b)
           end
           compile(model_def, (; b= 1))
       end
t: Error During Test at REPL[18]:1
  Got exception outside of a @test
  UndefVarError: `foo1` not defined in `Main`
...
```