Make the store directory a member variable of Store

Makefile

perl/lib/Nix/Store.xs

src/libexpr/eval.cc

src/libexpr/primops.cc

src/libstore/binary-cache-store.cc

src/libstore/binary-cache-store.hh

src/libstore/build.cc

src/libstore/builtins.cc

src/libstore/derivations.cc

src/libstore/derivations.hh

src/libstore/export-import.cc

src/libstore/gc.cc

src/libstore/http-binary-cache-store.cc

src/libstore/local-binary-cache-store.cc

src/libstore/local-fs-store.cc

src/libstore/local-store.cc

src/libstore/local-store.hh

src/libstore/nar-info-disk-cache.cc

src/libstore/nar-info-disk-cache.hh

src/libstore/nar-info.cc