This makes WINDOW_G a configurable value in the range of [2..24].
The upper limit of 24 is a defensive choice. The code is probably
correct for values up to 27 but those larger values yield in huge
tables (>= 256MiB), which are i) unlikely to be really beneficial
in practice and ii) increasingly difficult to test.
Identifiers starting with an underscore and followed immediately by a capital letter are reserved by the C++ standard.
The only header guards not fixed are those in the headers auto-generated from java.
This vastly shrinks the size of the context required for signing on devices with
memory-mapped Flash.
Tables are generated by the new gen_context tool into a header.
