feat: add support for platform specs (#15)

This PR adds a new way of declaring supported platforms for packages. Previously individual build, host, and target values needed to be set for a given platform combination. Now, a platform spec can be written to generate many platform combinations.

This allows for easy mapping of all possible systems with `@all`, linux systems with `@linux` or any other available alias of `lib.systems.doubles.<alias>` by using `@<alias>`. In addition, you can use `@build` and `@host` to lock either the host or target respectively to match the prior platform.

Reviewed-on: #15

tidepool/npins/sources.json

@@ -20,9 +20,9 @@
       },
       "branch": "main",
       "submodules": false,
-      "revision": "c55943c5e8197cbc2d0e506693fd5e7e3ea0a0c3",
+      "revision": "2af4e9f1d5dfe835c0eba15be1035bfa4065aca7",
       "url": null,
-      "hash": "sha256-XBhIQ0XR1QabTgyo2c5wMk6xk/Bd3bCUlnTzRPcVfcw="
+      "hash": "sha256-fwycuhXJGDHniLlxA8Xx4IELUObrP0EW3EnsDwNdyoQ="
     }
   },
   "version": 6