Refactor to return results by value, add wallet list_unspent and related types

README.md

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+
+
+Adding new structs and functions
+
+1. Create C safe Rust structs and related functions using safer-ffi
+
+2. Test generated library and `bdk_ffi.h` file with c language tests in `cc/bdk_ffi_test.c`
+
+3. Use `build.sh` and `test.sh` to build c test program and verify functionality and 
+   memory de-allocation via `valgrind` 
+
+4. Update the kotlin native interface LibJna.kt in the `bdk-kotlin` `jvm` module to match `bdk_ffi.h`
+
+5. Create kotlin wrapper classes and interfaces as needed
+
+6. Add tests to `bdk-kotlin` `test-fixtures` module 
+
+7. Use `build.sh` and `test.sh` to build and test `bdk-kotlin` `jvm` and `android` modules