Drop explicit preopens

Given WASI's CloudABI heritage I understand why preopens currently are the way that they are.

But they feel like something that should be just an host implementation detail. Ie. could the following also work?:

Remove

get-directories: func() -> list<tuple<descriptor, string>>

in exchange for:

instance-root-directory: func() -> descriptor

which returns a single directory that represents the "default filesystem". In similar vein to:

• wasi-clocks/instance-monotonic-clock
• wasi-clocks/instance-wall-clock
• wasi-network/instance-network

By default this is an empty directory. But the host can "mount" specific external directories/files into it. Exactly how the contents of this "filesystem" are populated (preopened or not), would be of concern only to the host, not the client application / libc.

Start by reading the issue discussion and compare the proposed instance-root-directory interface with the existing get-directories interface and the listed wasi-clocks and wasi-network instance interfaces. Done means reaching a maintainer-approved design for whether preopens should be hidden and clearly defining the proposed host/client contract.