Flesh out MemoryMount into a writable mount

This moves MemoryMount to the main core module, and converts it to be a
"proper" WritableMount. It's still naively implemented - definitely
would be good to flesh out our tests in the future - but enough for what
we need it for.

We also do the following:
 - Remove the FileEntry.path variable, and instead pass the path around
   as a variable.
 - Clean up BinaryReadableHandle to use ByteBuffers in a more idiomatic
   way.
 - Add a couple more tests to our FS tests. These are in a bit of an odd
   place, where we want both Lua tests (for emulator compliance) and
   Java tests (for testing different implementations) - something to
   think about in the future.

projects/lints/src/main/kotlin/cc/tweaked/linter/LoaderOverride.kt

@@ -23,7 +23,7 @@ internal object LoaderOverrides {
     private const val FABRIC_ANNOTATION: String = "dan200.computercraft.annotations.FabricOverride"
 
     fun hasOverrideAnnotation(symbol: Symbol.MethodSymbol, state: VisitorState) =
-        ASTHelpers.hasAnnotation(symbol, Override::class.java, state)
+        ASTHelpers.hasAnnotation(symbol, "java.lang.Override", state)
 
     fun getAnnotation(flags: ErrorProneFlags) = when (flags.get("ModLoader").orElse(null)) {
         "forge" -> FORGE_ANNOTATION