feat: drop typescript@3.0 support

Mapped tuples/arrays were causing a ton of head ache, so 3.1.1 is now the minimum supported version.

package.json

@@ -70,7 +70,7 @@
     "rollup-plugin-node-resolve": "^3.0.2",
     "rollup-plugin-uglify": "^2.0.1",
     "seamless-immutable": "^7.1.3",
-    "typescript": "3.0.1",
+    "typescript": "3.1.1",
     "uglify-es": "^3.3.6",
     "yarn-or-npm": "^2.0.4"
   },

readme.md

@@ -513,7 +513,7 @@ const newState = produce<State>(state, draft => {
 
 This ensures that the only place you can modify your state is in your produce callbacks. It even works recursively and with `ReadonlyArray`s!
 
-**Note:** TypeScript v2.x and lower are _not_ supported by Immer v1.9 and higher.
+**Note:** Immer v1.9+ supports Typescript v3.1+ only.
 
 ## Immer on older JavaScript environments?
 

yarn.lock

@@ -4483,10 +4483,10 @@ type-check@~0.3.2:
   dependencies:
     prelude-ls "~1.1.2"
 
-typescript@3.0.1:
-  version "3.0.1"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.0.1.tgz#43738f29585d3a87575520a4b93ab6026ef11fdb"
-  integrity sha512-zQIMOmC+372pC/CCVLqnQ0zSBiY7HHodU7mpQdjiZddek4GMj31I3dUJ7gAs9o65X7mnRma6OokOkc6f9jjfBg==
+typescript@3.1.1:
+  version "3.1.1"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.1.1.tgz#3362ba9dd1e482ebb2355b02dfe8bcd19a2c7c96"
+  integrity sha512-Veu0w4dTc/9wlWNf2jeRInNodKlcdLgemvPsrNpfu5Pq39sgfFjvIIgTsvUHCoLBnMhPoUA+tFxsXjU6VexVRQ==
 
 uglify-es@^3.3.6:
   version "3.3.9"