backend: add size of structures into refs file (<struct s=''> tag)
frontend: handle <struct s=''> tag - structure size in bytes
Drop unused SSIZE_STR_MAX macro
Add missed '/' for <struct> node
Add only zero-or-positive struct sizes into structure_sizes cache

I suppose that the next code is valid:
```
struct a { };
```
And size for this struct will be zero, that's why I keep >= 0.
change the format of the size in the refs/ file

To be consistant with the other entries.

Also there was a bug in the javascript that leads to show "undefined bytes"
when there is no size
Merge branch 'add-struct-size'