packages sharing immutable symbols without values

[DanGrayson]

Many packages introduce immutable symbols with capitalized names to use as names of optional arguments, and there is no reason such things need to be private to the package (they don't get values assigned). Perhaps we should add another dictionary to the path for symbols in common use, shared by packages, and ensure that the user gets a new warning when exporting such symbols that shadow another. There would be a variant of "export" called "exportShared" that puts such symbols into that new dictionary for all to share, or gets it from there, if it's already there.

[DanGrayson]

There should also be something similar for methods.

[DanGrayson]

We could also allow packages to create method functions, since typically packages install methods for different combinations of types.