[Cython] Lisp inspired transforms

[Martin C. Martin]

To recap the motivation for having it in base Cython:

- C++ template metaprogramming shows the value of compile time 
metaprogramming, i.e. writing programs that run at compile time and that 
produce programs as their output.

- However, C++ templates are essentially a separate, purely functional 
programming language embedded in C++, with different syntax and 
semantics than C+.  Of course, they were never really intended or 
designed for metaprogramming.  But now that they're being used that way, 
it's clear that the substitution model of templates is awkward and 
unnatural.  If we're going to write little functions to run at compile 
time, it makes sense to use the syntax and semantics of the base language.

Compile time metaprogramming doesn't exist in Python, so adding it to 
Cython means extending Cython beyond what Python has.  There are a 
couple options:

1. Add a way to generate C++ templates, and use that for 
metaprogramming.  It keeps Cython as "writing both Python and C++ with 
an extended Python syntax."

2. Don't add metaprogramming, even C++ templates.  Keeps Cython close to 
Python and C and C++, including all the good & bad parts.

3. Add a way to specify Python code for the transformation.  This 
recognizes that metaprogramming is a valuable activity that Cython 
developers will want to do; that the existing way to do it in C++ is 
more-or-less not up to the task; and that it's better to provide a new, 
cleaner mechanism using what we've learned in hindsight.

Of course, metaprogramming in an imperative, stateful language, opens a 
can of worms, e.g. it will be valuable to modify Python data at compile 
time, and have that serialized once all transforming is done, then 
loaded at the start of runtime.  I don't think any of these problems are 
particularly difficult though.

So, what do people this is the best way forward for Cython?

Best,
Martin