[kevinushey]

Fixes #1527. Stacked on #1526 (same macro); the base branch is bugfix/module-boot-exceptions, so the diff here only shows this change. Once #1526 is merged I'll retarget this to master.

Module("name", PACKAGE = ...) calls the module's boot function every time it is given a module name, and the boot function re-ran the RCPP_MODULE body each time. The body adds to a static Rcpp::Module that lives as long as the DLL, so every load appended another copy of each constructor and method overload, which shows up in show() and leaks. See #1527 for the reprex.

Change (inst/include/Rcpp/Module.h, RCPP_MODULE):

• A static bool flag next to the static Rcpp::Module records that the body has completed; later calls of the boot function skip the body and just return the module pointer. The flag lives in the same DLL instance as the module object, so a DLL that is unloaded and loaded again starts over with a fresh module and flag, as it should.
• The flag is only set after the body returns normally. If the body throws (now an R error, see turn exceptions during module initialization into r errors #1526), a later load runs it again. That can duplicate whatever was registered before the exception, but that is no worse than today, and the module was unusable anyway.

As with #1526, this is confined to the macro and applies once a package is rebuilt.

Test (inst/tinytest/test_module.R): after sourceCpp() has loaded demoModule, load it twice more with Module(..., mustStart = TRUE) and check that ModuleWorld still has one constructor and one greet() overload. It sits at the end of the file because Module() redefines the module's reference classes.

Verification:

• The reprex from each Module() call re-runs the module body, duplicating constructors and methods #1527 prints 2, 3, 4 constructors/overloads against master (and against turn exceptions during module initialization into r errors #1526), and 1 constructor(s), 1 overload(s) of inc() on every iteration with this branch; show() lists a single constructor and method.
• Without the change the new tests fail (Expected '1', got '3'). With it, test_module.R (49/49), test_modref.R, test_expose_class.R and test_module_client_package.R pass with RunAllRcppTests=yes RunVerboseRcppTests=yes (R 4.6.1, macOS).

Checklist

• Code compiles correctly
• R CMD check still passes all tests (module test files run locally as above)
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog