imgproc: refactor EMD to reduce C-API usage

[mshabunin]

• added more tests for EMD
• refactored to remove CvArr
• used BufferArea for memory allocations
• renamed functions and variables and formatted the code
• kept legacy functions intact in separate header

[opencv-alalek]

There are regressions in 32-bit builders:

• https://pullrequest.opencv.org/buildbot/builders/4_x_noOCL-lin32/builds/100626
• https://pullrequest.opencv.org/buildbot/builders/4_x_noICV-win32-vc14/builds/100072