Unify model construction workflow

[yebai]

Currently, model definition in JuliaBUGS is inconsistent:

• Models defined with @bugs or @bugs_str require an explicit call to compile(...).
• Models defined with @model are directly callable and return a BUGSModel.

This inconsistency can be confusing and complicates the implementation.

# @bugs macro
model1 = @bugs begin 
   x[1:2] ~ dmnorm(mu[:], sigma[:, :]) 
   x[3] ~ dnorm(0, 1) 
   y = x[1] + x[3] 
end

# @bugs_str macro
model2 = @bugs"""
  model {
     x[1:2] ~ dmnorm(mu[:], sigma[:, :]) 
     x[3] ~ dnorm(0, 1) 
     y = x[1] + x[3] 
  }
"""

# @model macro
@model function model3((;x), mu, sigma)
   x[1:2] ~ dmnorm(mu[:], sigma[:, :]) 
   x[3] ~ dnorm(0, 1) 
   y = x[1] + x[3] 
end

Usage:

# Requires compile(...)
compile(model1, (;mu=[0,0], sigma=[1 0; 0 1]))
compile(model2, (;mu=[0,0], sigma=[1 0; 0 1]))

# Directly callable
model3((;), [0,0], [1 0; 0 1])

Unify the API so that all models—whether defined via @bugs, @bugs_str, or @model—return a callable function of the form

model(data::NamedTuple) -> BUGSModel 

That is, let @bugs and @bugs_str return an anonymous function instead of a static object.

# unified behaviour
m1 = model1((;mu=[0,0], sigma=[1 0; 0 1])) # --> `compile(model1, (;mu=[0,0], sigma=[1 0; 0 1]))`
m2 = model2((;mu=[0,0], sigma=[1 0; 0 1])) # --> `compile(model2, (;mu=[0,0], sigma=[1 0; 0 1]))`

# The current `@model` generated constructor would still work. 
# But, this is not available for models defined via `@bugs` and `@bugs_str` 
m3 = model3((;), [0,0], [1 0; 0 1])

# all models can be initialised via
initialize!(::BUGSModel, initial_params::NamedTuple)

Then, we can remove compile from the export list and keep it only as an internal utility.