[DRAFT] Update linker to enable cross-language C/Rust linking

[danielsn]

Currently, the CBMC linker takes a fairly strict interpreation of compabibility between compilation units. Rust has a more relaxed view when doing FFI linking: two types are identical if they have the same bitpattern, even if they appear different at the surface level.

For example, the Rust linker would consider the following types equivalent:

// empty struct
struct Unit {};

//These are both considered equivilent to uint32_t*
struct OptionRef { uint32_t* p;};
struct OptionRefWithEmptyFields { struct Unit u; uint32_t* p;};

Update the linker to make this work

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[kroening]

Linking may have very language-specific aspects. It's not obvious that we want to have one linker that can link just about any language. In particular, a particular idiom may be well permitted in one language, and a bug in another.

[martin-cs]

@danielsn I am cautiously optimistic about this. It would be really cool if we could do this but I am a bit worried whether we can capture the necessary details involved in FFI and cross-language linking.

Paging @tautschnig who has spent way more time thinking about linking than me.