Guest function improvements and macros

[jprendes]

This pull request introduces a new func module to the hyperlight-common crate, which is used to providing new register_fn function for guest function registration: register_fn("function_name", the_function). The traits in the func module are used to specify and constrain the type of the_function.

It refactors existing function-related code from hyperlight_host into hyperlight_common.

This pull request also introduces a new hyperlight_guest_macro crate that defines two proc-macros: host_function and guest_function.

• host_function: This macro is used to declare host functions in a hyperlight guest. The specified function must have no body (like an ffi function declaration), and the signature is used to create a wrapper for a host function call with the correct types.
• guest_function: This macros is used to declare guest functions in a hyperlight guest. The function signature is used to create a wrapper that takes care of extracting the arguments from the FunctionCall and serializing the result using into_flatbuffer_result. The function is also automatically registered before the call to hyperlight_main, removing the need to do manual registration in hyperlight_main.

To enable the automatic registration of functions, the MSRV is bumped to 1.89 (which includes rust-lang/rust#137685, which is required as the automatic registration uses linkme)

[ludfjig]

Looks good to me. Btw, how does linkme work? xD

[jprendes]

Looks good to me. Btw, how does linkme work? xD

This is the best explanation I found, other than inspecting the source code :-)
https://internals.rust-lang.org/t/from-life-before-main-to-common-life-in-main/16006/24

[dblnz]

This looks great! Good work Jorge!
I didn't get to look very thoroughly, but overall it looks good