8a1b7965d8
Previously, "_start" was run as part of module instantiation, which meant it was always run, even for wasm modules that weren't being loaded as commands. Now, just invoke it from the wasmtime driver, which for now is the only place that runs wasm modules as actual commands. Also, stop recognizing the old "main" entry point, which tools have stopped using a while ago, and switch to start recognizing the "" entrypoint.
This is the wasmtime-runtime crate, which contains wasm runtime library
support, supporting the wasm ABI used by wasmtime-environ,
wasmtime-jit, and wasmtime-obj.
This crate does not make a host vs. target distinction; it is meant to be compiled for the target.