Ramda: Fix splitAt signature

- `splitAt` accepts either arrays or strings, not any arbitrary T.
- the output will always be an array of two elements, so switched to
  using the tuple syntax there.
- ensures that the overloading after first partial application works

http://ramdajs.com/docs/#splitAt

fixup splitat

types/ramda/index.d.ts

@@ -1644,10 +1644,12 @@ declare namespace R {
         /**
          * Splits a given list or string at a given index.
          */
-        splitAt<T>(index: number, list: T): T[];
-        splitAt(index: number): <T>(list: T) => T[];
-        splitAt<T>(index: number, list: ReadonlyArray<T>): T[][];
-        splitAt(index: number): <T>(list: ReadonlyArray<T>) => T[][];
+        splitAt<T>(index: number, list: ReadonlyArray<T>): [T[], T[]];
+        splitAt(index: number, list: string): [string, string];
+        splitAt(index: number): {
+            <T>(list: ReadonlyArray<T>): [T[], T[]];
+            (list: string): [string, string];
+        };
 
         /**
          * Splits a collection into slices of the specified length.

types/ramda/ramda-tests.ts

@@ -1899,6 +1899,7 @@ class Rectangle {
     const b: number[][] = R.splitAt(1)([1, 2, 3]);        // => [[1], [2, 3]]
     const c: string[]   = R.splitAt(5, "hello world");      // => ['hello', ' world']
     const d: string[]   = R.splitAt(-1, "foobar");          // => ['fooba', 'r']
+    const e: string[]   = R.splitAt(-1)("foobar");          // => ['fooba', 'r']
 };
 
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