[io][fidl] Update io.fidl to comply with the FIDL style guide

As documented:
https://fuchsia.googlesource.com/docs/+/master/development/api/fidl.md

- Constants must be named in ALL_CAPS_SNAKE_CASE.
- Enum members must be named in ALL_CAPS_SNAKE_CASE.

Test: CQ

Change-Id: I643e0aec420c3c61753fad15a03a79f6f4fa9d52

[fdio][fidl] Use unaligned dirents for readdir, add ino

Test: fs-tests
ZX-2502 #done

Change-Id: I93e932a629f25c7da9f9a1a06e022d907ada47ed

[fs] Create LazyDir, which is populated dynamically

PseudoDir implements a relatively static directory consisting of entries
backed by ordered trees. A LazyDir, by contrast, is not backed by any
storage at all. Each operation on a LazyDir refreshes the view of the
directory.

LazyDir is meant as a base class for directories with rapidly changing
contents, and derived classes implement GetContents and GetFile callback
methods to expose files (or further directories) in the directory.

Future work will consist of a caching mechanism to improve benchmark
performance, particularly on deeply nested LazyDirs.

TEST=zircon$ runtests -t fs-vnode-test

ZX-2447: #done
Change-Id: I2c3465f67fcd9b99441912554862b37b290d84e3