'Enum' has a big performance impact on the running code. Replacing
it with a safe subset of functionality removes lots of this overhead
without removing the benefits of using enums (safe comparisions,
uniqueness)

`url_directory_name` is heavily called from any downloadable source
plugin, and moving it to cython gives a more than 10x speedup

This makes the 'Node' API public, and available for use directly for
plugins.

- _yaml: export node_validate function as Cython, as it was not done
before. This requires rewriting the function to remove a closure.
- Optimize node check by not calling is_node().

Types is a simple module that accounts for a few percent of a basic
'show' operation. Having it cythonized allows us to get better
performance without too much wokr

- Create a new _loader/utils.pyx cython module for functions cythonized
in the loader module.

- Move valid_chars_name from loader to utils and cythonize.

This function is called extensively, and easy to extract

"0" evaluates to 'True' in python, which incorrectly switched on
the BST_CYTHON_TRACE. Forcing an int for this environment variable
allows us to ensure we are correct.

It turns out that enabling 'linetrace', does have a runtime cost even
if not enabled in distutils.

Therefore disabling it by default. In order to run coverage, we
therefore need to retranspile the .pyx files with ENABLE_CYTHON_TRACE
set.