Add DRC checks for creepage

Description

At a minimum this would be clearance check with additional smarts to go around board/cutout/hole edges in the X/Y and Z planes.

Ideally it would also:

• handle grooves and/or notches.
• support some sort of scaling for dielectric/coating/etc.

KiCad Version

Application: Pcbnew

Version: (5.99.0-2681-gd97e51903-dirty), debug build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.57.0 OpenSSL/1.0.2n zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4)

Platform: macOS Mojave Version 10.14.6 (Build 18G6020), 64 bit, Little endian, wxMac

Build Info:
	Date: Aug 12 2020 11:21:23
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.72.0
	Curl: 7.57.0
	ngspice: 26
	Compiler: Clang 11.0.0 with C++ ABI 1002

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=OFF
	KICAD_SCRIPTING_PYTHON3=OFF
	KICAD_SCRIPTING_WXPYTHON=OFF
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
	KICAD_SCRIPTING_ACTION_MENU=ON
	BUILD_GITHUB_PLUGIN=ON
	KICAD_SPICE=ON
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE=OFF