* "Fix expression" -> "Nix expression".

src/Makefile.am

src/bin2c/Makefile.am

src/bin2c/bin2c.c

src/nix-instantiate/Makefile.am

src/nix-instantiate/eval.hh

src/nix-instantiate/main.cc

src/nix-instantiate/nix.sdf

src/nix-instantiate/nixexpr.cc

src/nix-instantiate/nixexpr.hh

src/nix-instantiate/parser.cc

src/nix-instantiate/parser.hh

src/nix-store/Makefile.am

src/nix-store/help.txt

src/nix-store/main.cc