Templates for defining data types/etc

WIP support for 010-style Binary Templates is now working on the binary-template branch.

Still missing lots of functionality, but any relatively simple template will probably work.

Things that still need doing:

[x] Implement distinct number types (int/double, signed/unsigned, clamping values to type limits)
[x] Make string and char[] interchangable
[x] Reference types as function parameters
[x] Increment (++) and decrement (--) operators
[x] do-thing-and-assign shorthand operators (+=, -=, *=, etc)
[ ] Write a manual...
[ ] Integration tests for preprocessor/parser/executor
[x] Clean up error reporting in executor
[x] Don't cross struct boundaries when resolving variables (and types?)
[x] Add to CI coverage report
[x] Scope variables/types within blocks
[x] Don't allow function declarations inside other blocks
[ ] Tidy up UI
[x] Allow operating on a range/selection rather than the whole file
[x] Run template in a transaction
[x] Yield when reading huge amounts of file data
[x] Ternary operator
[x] Parse x-1 as (x) (-) (1) rather than (x) (-1)
[x] Hex and octal numbers
[x] Populate array from a loop syntax
[x] Declare anonymous enum variables
[x] Fix random failures in parser...
[x] Make array_extend() on empty file arrays work (don't depend on data_range)
[ ] Add template output window
[ ] Remember templates selected by "Browse" button

Things I'm probably not going to support in the first version:

Things I'm not going to support:

Expect all of the above lists to grow.

solemnwarning / rehex