Eliminate the need for @load_generated_functions.

[ztangent]

This PR eliminates the need to call @load_generated_functions in order to make use of a generative function written in the static modeling language. Instead, new @generated GFI methods (simulate, update, etc.) are defined for each static generative function. These GFI definitions are evaluated after the get_ir and get_options definitions returned by Gen.generate_generative_function, preventing world-age issues.

Besides making the static modeling language easier to use, this change prevents incremental precompilation warnings and breakages which were occurring when multiple sub-packages called @load_generated_functions, of the form:

WARNING: Method definition simulate(Gen.StaticIRGenerativeFunction{T, U} where U where T, Tuple)
in module TestSubPkgA at C:\...\Gen.jl\src\static_ir\simulate.jl:86 overwritten in module TestSubPkgB
on the same line (check for duplicate calls to `include`).
  ** incremental compilation may be fatally broken for this module **

Minimal working example: TestPkg.zip

This warning occurs because @generated function simulate(...) was being defined multiple times:

1. Once when some dependencyTestSubPkgA that calls @load_generated_functions is precompiled.
2. Again where another dependency TestSubPkgB that calls @load_generated_functions is precompiled.

The second definition overwrites the first definition for the exact same @generated method, leading to precompilation warnings. This PR addresses the issue by defining a different @generated method for each new generative function.

In addition to the main change:

• All tests have been updated to remove calls to load_generated_functions or @load_generated_functions.
• Deprecation warnings have been added to the docstrings and function bodies of those methods
• Documentation has been updated to reflect that calling @load_generated_functions is no longer necessary.

For documentation and docstrings, we may want to specify a version number, e.g. Gen v0.4.6, for when load_generated_functions is no longer necessary. But apart from that, I'm pretty sure this is safe for the rest of the Gen ecosystem!

[femtomc]

LGTM! A nice straightforward fix!