AK: Add FixedMemoryStream methods for reading values "in place"

When working with FixedMemoryStreams, and especially MappedFiles, you
may don't want to copy the underlying data when you read from the
stream. Pointing into that data is perfectly fine as long as you know
the lifetime of it is long enough.

This commit adds a couple of methods for reading either a single value,
or a span of them, in this way. As noted, for single values you sadly
get a raw pointer instead of a reference, but that's the only option
right now.

Tests/AK/TestMemoryStream.cpp

@@ -243,3 +243,30 @@ TEST_CASE(fixed_memory_truncate)
 
     EXPECT(stream.truncate(999).is_error());
 }
+
+TEST_CASE(fixed_memory_read_in_place)
+{
+    constexpr auto some_words = "These are some words"sv;
+
+    FixedMemoryStream readonly_stream { ReadonlyBytes { some_words.bytes() } };
+
+    // Trying to read mutable values from a read-only stream should fail.
+    EXPECT(readonly_stream.read_in_place<u8>(1).is_error());
+    EXPECT_EQ(readonly_stream.offset(), 0u);
+
+    // Reading const values should succeed.
+    auto characters = TRY_OR_FAIL(readonly_stream.read_in_place<u8 const>(20));
+    EXPECT_EQ(characters, some_words.bytes());
+    EXPECT(readonly_stream.is_eof());
+
+    FixedMemoryStream mutable_stream { Bytes { const_cast<u8*>(some_words.bytes().data()), some_words.bytes().size() }, true };
+    // Trying to read mutable values from a mutable stream should succeed.
+    TRY_OR_FAIL(mutable_stream.read_in_place<u8>(1));
+    EXPECT_EQ(mutable_stream.offset(), 1u);
+    TRY_OR_FAIL(mutable_stream.seek(0));
+
+    // Reading const values should succeed.
+    auto characters_again = TRY_OR_FAIL(mutable_stream.read_in_place<u8 const>(20));
+    EXPECT_EQ(characters_again, some_words.bytes());
+    EXPECT(mutable_stream.is_eof());
+}