Dynamically compile spec'd __init__ functions

[jparise]

init_func_generator() previously used a single argument-less init base
function coupled with a locals() hack to build spec-aware __init__()
functions. This worked well, but profiling indicated that we were spending a
good amount of time calling locals() and dict.copy()'ing its result.

We now dynamically generate unique, per-spec __init__() functions using
compile(..., 'exec'). This results in much faster runtime performance at the
expense of a little compile-time complexity.

The benchmark indicates this change results in ~10% faster decoding (for both
the native and cybin implementations).

Also, because this code no longer requires version-specific code paths, it can
be moved out of _compat.py and into thrift.py.

[jparise]

Alternatively, we could construct the function's AST by hand (instead of compiling a code string). That seemed like it would be more work than this, but I can explore that option if it would be preferred.

[lxyu]

Thanks! Happy to see performance improvements, and I personally like the AST option and I'd be happy to merge it if you have time to explore it.

[jparise]

@lxyu, I spent a little bit of time on the AST approach, and I'm afraid it may be more complicated than I had expected. Building the function call isn't that hard, but there isn't an "easy" way to generate the typed default values for the keyword arguments (e.g. Str() vs. Num() vs Name(id='None', ctx=Load())).

[lxyu]

@jparise 👍, thank you very much for your effort, let's go on with the current solution then.