Fixes for binding against libagc

[pjotrp]

Added shared library as output, i.e.

make libagc_so

Also fixes C ABI:

1. Prevent C++ exceptions to bleed through the C ABI which causes Rust bindings to fail by handling 'noexcept'
2. Added missing agc_string_destroy in ABI

[pjotrp]

Note I create Rust bindings linked against libagc.so:

=> https://github.com/pjotrp/libagc-sys

See also the included test in lib.rs. It binds against the updated agc:

=> main...pjotrp:agc:main

which contains the fixes that don't allow C++ exceptions to pass through the C ABI.
Also I fixed one function and added a shared lib as output.

Finally, rather than messing with the impg code tree (which keeps changing), I created a test crate that mirrors impg:

=> https://github.com/pjotrp/testlibagc

which can be build and run with

cargo build --release
target/release/testagc-sys
Number of samples: 4

At least we have a reference implementation for binding successfully against a shared C library with a very light and standardised interface. It obviously also works in Guix. We can use it to benchmark against the new (impressive) Rust implementation by Erik. It also acts as a template for future bindings.