feat(nat/int/kernel/subtype): Give definition of int based on quotients and make other minor changes

Minor changes in other files: Add some more theorems about tprod in kernel (and some cleanup of long lines); move some code from nat2 to nat; add thm to subtype and changed (Type U) to Type.
chore(library/.gitignore): move to 'library/', add 'make.deps'
chore(library/standard/subtype.lean): change permission
chore(library/Makefile.common): use structured Makefile

 - 'library/Makefile.common' has the common features
 - 'library/standard/Makefile' and 'library/hott/Makefile' just include
   'library/Makefile.common'
 - simplify extract_deps and put it under library/
feat(library/standard/nat2): add a mechanism for a general recursive definition with respect to a nat-valued measure