This change fixes the suite names all around the test files, to make them match to the folder name in which this test files are located. Also, the RCL test files are relocated to the consensus folder, because they are testing consensus functionality.
Adds two CMake functions:
* add_module(library subdirectory): Declares an OBJECT "library" (a CMake abstraction for a collection of object files) with sources from the given subdirectory of the given library, representing a module. Isolates the module's headers by creating a subdirectory in the build directory, e.g. .build/tmp123, that contains just a symlink, e.g. .build/tmp123/basics, to the module's header directory, e.g. include/xrpl/basics, in the source directory, and putting .build/tmp123 (but not include/xrpl) on the include path of the module sources. This prevents the module sources from including headers not explicitly linked to the module in CMake with target_link_libraries.
* target_link_modules(library scope modules...): Links the library target to each of the module targets, and removes their sources from its source list (so they are not compiled and linked twice).
Uses these functions to separate and explicitly link modules in libxrpl:
Level 01: beast
Level 02: basics
Level 03: json, crypto
Level 04: protocol
Level 05: resource, server
Remove the implicit conversion from int64 to XRPAmount. The motivation for this
was noticing that many calls to `to_string` with an integer parameter type were
calling the wrong `to_string` function. Since the calls were not prefixed with
`std::`, and there is no ADL to call `std::to_string`, this was converting the
int to an `XRPAmount` and calling `to_string(XRPAmount)`.
Since `to_string(XRPAmount)` did the same thing as `to_string(int)` this error
went undetected.
