Refactor TL2 codegen

[JCGoran]

The codegen for TL2 was a bit difficult to reason about since the C++ code was directly embedded as a Python string. With this PR, I've added a simple Jinja2 template which contains the whole file, making it much easier to follow as well as make changes.

Other notable changes:

• added jinja2 to requirements
• don't use defines but constexpr auto instead
• some of the pragma unroll loops have been made portable (notably to GCC) using the UNROLL_LOOP macro (see also #83)

The generated bitnet-lut-kernels.h is more or less identical (with some slight whitespace differences).

Note that I didn't do the same for TL1 codegen since I'd first like to get feedback on whether this is a step in the right direction.