This commit adds support for snapshot0 in the WASI C API.
A name parameter was added to `wasi_instance_new` to accept which WASI module
is being instantiated.
Additionally, the C# API now supports constructing a WASI instance based on the
WASI module name.
Fixes#1221.
This commit renames `wasi_config_set_std[in|out|err]` to
`wasi_config_set_std[in|out|err]_file` so we can reserve the former for
when the C API supports a stream abstraction.
This commit implements an initial WASI C API that can be used to instantiate
and configure a WASI instance from C.
This also implements a `WasiBuilder` for the C# API enabling .NET hosts to bind
to Wasmtime's WASI implementation.