Streaming reading in wasmparser

I looked at wasmparser and naively expected the API to work with a `std::io::BufReader` as input, rather than requiring the full file contents up front.

Is there something fundamental to the WASM format that precludes this? Was it an intentional design decision here to not use such a thing? Glancing at `binary_reader`, it seems it works with a cursor-like concept anyway.

Start by reading wasmparser's binary_reader and the current input API to understand its cursor-like assumptions and whether BufReader is compatible. Determine whether streaming requires a design change or is precluded by the WASM format; document the conclusion and the intended API direction.

Written by the indexing model from the issue text.