Add support for multiple arguments

[ModProg]

Currently, in extism calling plugin functions can only take a single argument.

I'd like macros like typed_plugin and plugin_fn to support multiple arguments, they would just convert them to use tuples under the hood.

IMO this could be implemented through ToBytes and FromBytes for Tuples where the individual fields implement those types.

This implementation could work as follows:

1. () already does
2. (T,) can just delegate to T
3. (T1, T2, ...) add one extra usize (could possibly be a different integer type) per item. The memory layout of the bytes array produced by tuples would need to be <T1::to_bytes().len()> <T1::to_bytes()> <T2::to_bytes().len()> <T2::to_bytes()> ....

I'd imagine this would work quite well for rust, but I'm uncertain for other languages, e.g., such without an actual tuple type.