Remove system-interface from wasmtime-wasi

[alexcrichton]

This commit removes the system-interface dependency from wasmtime-wasi, reimplementing necessary functionality within the crate itself as appropriate. The goal of this commit is to trim our dependency tree. The system-interface crate has not received an update in over a year and continues to pull in an older rustix dependency for example. Additionally I've personally found it confusing and surprising in the past to trace through all the layers of abstractions from wasmtime-wasi to the OS, and I'd like to start slimming this down to be more local within Wasmtime rather than depending on a tree of crates.

The system-interface crate is a relatively thin wrapper around cap-std-style crates providing a platform-agnostic API. This sometimes fits what WASI wants, and sometimes doesn't. For example all reads/writes to files within WASI currently require that Wasmtime maintains a file cursor itself meaning that the underlying OS file cursor doesn't actually matter. Reads and writes through the system-interface abstraction, however, keep the cursor up-to-date to have the same semantics across Unix and Windows which differ in the behavior of the underlying syscalls. This is unnecessarily adds overhead to Wasmtime's implementation of these APIs where they're otherwise not required.

Effectively system-interface is not receiving much maintenance (old dependency on rustix has persisted for ~1 year), its an extra layer of abstraction to debug when issues arise, and its abstractions are not always the best fit for WASI's semantics meaning that it can add performance overhead. The replacement of inlining implementations within Wasmtime is not too too costly and, personally, I view as worth it.

I'll note that this doesn't delete the system-interface crate entirely. That would require removing it from wasi-common as well, which is the subject of #13108.