MSVC: compilation works again (+minor style fixes)

It's mainly moving variable declarations at the beginning of a block. I also took the liberty to:
- add the missing short description and reference to the copyright notice to the new header
- remove ```const``` from integer variables like ```uint32_t```

(The upside is very minimal and even then the declaration and definition should match (see the function ```mrb_debug_get_line``` in ```src/debug.c``` and ```include/mruby/debug.h```))
- adjust the style (casting void pointers, const placement), although not fully