Co-authored-by: Dentomologist <dentomologist@gmail.com>

window.

This is clearer and reduces IntelliSense problems.

This is mainly for debugging, and is only exposed by manually editing the configuration.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

This isn't used anywhere, so it can be removed. This also potentially
fixes an underlying compilation error waiting to happen, given DECSTAT
could have potentially been used, someone disables statistics (for
whatever reason), then gets a compilation error due to the #else case
not containing an empty definition of DECSTAT.

Makes the global variable follow our convention of prefixing g_ on
global variables to make it obvious in surrounding code that it's not a
local variable.

Rather than making Statistics' member functions operate on the global
variable instance of itself, we can make these functions member
functions and operate on a by-instance state, removing the direct
dependency on the global variable itself.

This also makes for less reading, as there's no need to repeat "stats."
for all variable accesses.