Merge wpilibc and wpilibj subprojects into wpilib

[calcmogul]

It's a C++ project, not a C project.

[PeterJohnson]

It would be better to merge both wpilibc and wpilibj into a single wpilib subproject for consistency with other subprojects (e.g. commands has both, even though C++ and Java are independent within it). We'll want to figure out some Gradle convenience targets to make it easy to build just the C++ or Java version, however, as :build builds everything.

[Gold872]

I know there was also the idea to change the java package from edu.wpi.first.wpilibj to something shorter like org.wpilib, if we want to make a change like that it might make sense to do it while merging the 2 subprojects