Some details in 'lmem.c' and 'lgc.c'

- Several new comments in 'lmem.c'.
- Both 'luaM_growaux_' and 'luaM_shrinkvector_' use 'luaM_saferealloc_'
  to check for errors. Moreover, the use of 'luaM_saferealloc_' makes
  'luaM_shrinkvector_' try again if shrink fails (which can happen now).
- In 'checkSizes', save old debt only when needed.
2 files changed