format strings for library and xbps-query

[Duncaen]

I'm not final on the api, don't really like the function names but couldn't come up with anything better.

Format strings are inspired by pythons f"" strings:

$ xbps-query -F '{pkgname:20}\n' -m | head
               Carla
         ImageMagick
        LuaJIT-devel
             MEGAcmd
              PolyMC
          SDL2-32bit
          SDL2-devel
            SDL2_gfx
      SDL2_gfx-32bit
          SDL2_image
$ xbps-query -F '{pkgver:-30} {installed_size:10h}\n' -m | head
Carla-2.5.1_2                        79MB
ImageMagick-7.1.0.62_1              398KB
LuaJIT-devel-2.1.0beta3_2           898KB
MEGAcmd-1.4.0_1                    1300KB
PolyMC-6.3_1                           0B
SDL2-32bit-2.26.3_1                1841KB
SDL2-devel-2.26.3_1                6150KB
SDL2_gfx-1.0.4_2                     72KB
SDL2_gfx-32bit-1.0.4_2               83KB
SDL2_image-2.6.2_1                  127KB
$ xbps-query -F '0x{installed_size:8zX}\n' -m | head
0x04F62C86
0x000636AA
0x000E08BA
0x0014512A
0x00000000
0x001CC564
0x006019F2
0x00012075
0x00014D9C
0x0001FA1D