Dynamic dispatch with supertraits

[liamnaddell]

There was only one function that needed modification, the rest is testing for this new feature :)

    gcc/rust/ChangeLog:

            * backend/rust-compile.cc:
            Modify compute_address_for_trait_item to support supertraits
            * typecheck/rust-tyty.cc:
            Remove auto

    gcc/testsuite/ChangeLog:

            * rust/compile/trait13.rs:
            Add test for supertraits of supertraits
            * rust/compile/trait14.rs:
            Diamond problem with supertraits test
            * rust/execute/torture/trait14.rs:
            Add test for dynamic dispatch with supertraits
            * rust/execute/torture/trait15.rs:
            Add test for dynamic dispatch with generics
            * rust/execute/torture/trait16.rs:
            Add test for dynamic dispatch with lifetime params 1
            * rust/execute/torture/trait17.rs:
            Add test for dynamic dispatch with lifetime params 2
            * rust/execute/torture/trait18.rs:
            Add test for default implementations with dynamic dispatch and
            supertraits

Fixes #914

• [x] GCC development requires copyright assignment or the Developer's Certificate of Origin sign-off, see https://gcc.gnu.org/contribute.html or https://gcc.gnu.org/dco.html
• [x] Read contributing guidlines
• [x] make check-rust passes locally
• [x] Run clang-format
• [x] Added any relevant test cases to gcc/testsuite/rust/

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Modifies compute_address_for_trait_item to support supertraits. The algorithm works by looping over receiver_bounds for all relevant traits, and attempting to find one that matches the type, and contains the correct function implementation.