Your code is impressingly well structured and easy to follow, thanks to its comprehensive documentation.
The first thing I noticed that I would do differently is when initializing the space character. Your code reads:
> 20 space >[-]++[-<++++++++++++++++>]<
You can leave out the [-] since the memory is zeroed out when the ...
The way you're handling add "modes" is a little clunky. It seems that only one of _add or _sub should ever be true at the same time. By making these two separate variables, you need to make sure that invariant is maintained everywhere. You've definitely helped make sure it's safe by using the add and sub functions, but those are band-aids on an awkward ...
For your first Go program this looks very good, apart from the global
variables the code looks easy to read; the annotations for JSON
processing and validation are fine and you're also using a standard
library for the routing. Overall, doesn't raise any big issues for me.
Firstly though, I'd highly suggest that the trades and submitArray
global variables ...