[SharedCache] Avoid copying strings on each call to VM::MappingAtAddress

`PageMapping` was storing the path to the file it points within. This
was causing unnecessary work within `VM::MappingAtAddress` as the
`PageMapping`, and thus the path, is copied into the return value. This
copying was more expensive than the map lookup.

The file path is now stored within a `LazyMappedFileAccessor` class that
wraps the `SelfAllocatingWeakPtr`. This means the path is still
available via the `PageMapping`, but it does not need to be copied as
often.

This includes two additional improvements / optimizations while I was
touching the code in question:
1. `MMappedFileAccessor::Open` no longer performs two hash lookups when
   a file accessor already exists.
2. `VM::MapPages` takes the path by const reference to avoid an
   unnecessary copy.

0 files changed, 0 insertions, 0 deletions