add allSystems (#7)

The flake design makes it unecessary hard to adopt new platforms
by hardcoding system. By having this list ready to use, I hope
people will write more portable flakes.

README.md

@@ -10,9 +10,13 @@ flakes.
 
 ## Usage
 
+### `allSystems -> [<system>]`
+
+A list of all systems defined in nixpkgs. For a smaller list see `defaultSystems`
+
 ### `defaultSystems -> [<system>]`
 
-A list of all the systems supported by the nixpkgs project.
+The list of systems supported by nixpkgs and built by hydra.
 Useful if you want add additional platforms:
 
 ```nix
@@ -30,6 +34,16 @@ Eg:
 ```nix
 eachSystem ["x86_64-linux"] (system: { hello = 42; })
 # => { hello.x86_64-linux.hello = 42; }
+eachSystem allSystems (system: { hello = 42; })
+# => {
+   hello.aarch64-darwin = 42,
+   hello.aarch64-genode = 42,
+   hello.aarch64-linux = 42,
+   ...
+   hello.x86_64-redox = 42,
+   hello.x86_64-solaris = 42,
+   hello.x86_64-windows = 42
+}
 ```
 
 ### `eachDefaultSystem -> (<system> -> attrs)`

default.nix

@@ -14,6 +14,55 @@ let
     "x86_64-linux"
   ];
 
+  # List of all systems defined in nixpkgs
+  # Keep in sync with nixpkgs wit the following command:
+  # $ nix-instantiate --json --eval --expr "with import <nixpkgs> {}; lib.platforms.all" | jq
+  allSystems = [
+    "aarch64-linux"
+    "armv5tel-linux"
+    "armv6l-linux"
+    "armv7a-linux"
+    "armv7l-linux"
+    "mipsel-linux"
+    "i686-cygwin"
+    "i686-freebsd"
+    "i686-linux"
+    "i686-netbsd"
+    "i686-openbsd"
+    "x86_64-cygwin"
+    "x86_64-freebsd"
+    "x86_64-linux"
+    "x86_64-netbsd"
+    "x86_64-openbsd"
+    "x86_64-solaris"
+    "x86_64-darwin"
+    "i686-darwin"
+    "aarch64-darwin"
+    "armv7a-darwin"
+    "x86_64-windows"
+    "i686-windows"
+    "wasm64-wasi"
+    "wasm32-wasi"
+    "x86_64-redox"
+    "powerpc64le-linux"
+    "riscv32-linux"
+    "riscv64-linux"
+    "arm-none"
+    "armv6l-none"
+    "aarch64-none"
+    "avr-none"
+    "i686-none"
+    "x86_64-none"
+    "powerpc-none"
+    "msp430-none"
+    "riscv64-none"
+    "riscv32-none"
+    "vc4-none"
+    "js-ghcjs"
+    "aarch64-genode"
+    "x86_64-genode"
+  ];
+
   # eachSystem using defaultSystems
   eachDefaultSystem = eachSystem defaultSystems;
 
@@ -71,6 +120,7 @@ let
 in
 {
   inherit
+    allSystems
     defaultSystems
     eachDefaultSystem
     eachSystem