Make String.split/Slice.split more consistent

Different languages take different approaches for how string splitting
should behave. On Discord Tomoki Aonuma (@uasi) pointed out some
odd/inconsistent behavior of our String.split/Slice.split implementation
compared to other languages.

This commit changes the behavior so it's more consistent with Rust. That
is:

1. We generate a slice for each separator, instead of skipping the last
   empty slice
2. An empty input produces a single empty slice

Changelog: changed

Author: yorickpeterse

std/src/std/bytes.inko

@@ -377,6 +377,8 @@ type pub inline Slice[T: Bytes] {
   # If the separator isn't present in `self`, the returned iterator yields
   # `self` exactly once.
   #
+  # If `self` is empty then a single empty `String` is yielded.
+  #
   # # Examples
   #
   # ```inko
@@ -395,7 +397,7 @@ type pub inline Slice[T: Bytes] {
     Stream.new(fn move {
       match index_of(separator, starting_at: off) {
         case Some(i) -> Option.Some(slice(off := i + sep_len, i))
-        case _ if off < len -> Option.Some(slice(off := len, len))
+        case _ if off <= len -> Option.Some(slice(off := len + 1, len))
         case _ -> Option.None
       }
     })

std/src/std/net/http/cookie.inko

@@ -417,7 +417,8 @@ type pub Cookie {
     input: String,
   ) -> Result[Array[Cookie], ParseError] {
     input.split(';').try_reduce([], fn (cookies, slice) {
-      cookies.push(try Cookie.parse_name_value(slice))
+      if slice.size > 0 { cookies.push(try Cookie.parse_name_value(slice)) }
+
       Result.Ok(cookies)
     })
   }

std/test/std/test_string.inko

@@ -189,12 +189,13 @@ fn pub tests(t: mut Tests) {
     t.equal(split('foo//bar/baz', on: '/'), ['foo', '', 'bar', 'baz'])
     t.equal(split('foo/bar/baz', on: ''), ['foo/bar/baz'])
     t.equal(split('foo/bar', on: '///////////////'), ['foo/bar'])
-    t.equal(split('foo/', on: '/'), ['foo'])
-    t.equal(split('foo//', on: '/'), ['foo', ''])
-    t.equal(split('foo///', on: '/'), ['foo', '', ''])
-    t.equal(split('foo//', on: '//'), ['foo'])
+    t.equal(split('foo/', on: '/'), ['foo', ''])
+    t.equal(split('foo//', on: '/'), ['foo', '', ''])
+    t.equal(split('foo///', on: '/'), ['foo', '', '', ''])
+    t.equal(split('foo//', on: '//'), ['foo', ''])
     t.equal(split('foo///', on: '//'), ['foo', '/'])
-    t.equal(split('', on: '/'), [])
+    t.equal(split('/foo//', on: '/'), ['', 'foo', '', ''])
+    t.equal(split('', on: '/'), [''])
   })
 
   t.test('String.split_once', fn (t) {