Python API cleanup for 1.0

Multiple tasks to clean up parts of the pyOCD Python API in preparation for a 1.0 release. Only potentially compatibility-breaking changes are listed here.

• need to make MemoryMap.get_first_region_of_type() exclude non-default regions, or add a get_default_region_of_type()
• rename ResetType.SW_SYSRESETREQ and SW_VECTRESET to SW_SYSTEM and SW_CORE
• split classes in flash_loader.py out into separate files
• add Target API to get the halt reason (i.e., breakpoint, vector catch, user halt request, etc)
• clean up contains related methods on MemoryRangeBase
• move constants defined in Target out of the class and into enums
• cleanup CortexM constants
• cleanup CortexM registers
• make read regs return None if it can't read a register (e.g. core is running)
• rename FlashReaderContext to ElfReaderContext
• remove ELFSymbolProvider and make ElfSymbolDecoder a subclass of SymbolProvider?
• standardize naming of acryonyms in symbols; ELF vs Elf, GDB vs Gdb, STLink vs Stlink, etc
• break out regcache, memcache, cache context and gdb packet and server into their own files for logging purposes so child loggers don't have to be created
• change CortexM.available_breakpoint_count property to function that takes bp type, calls bpmgr function
• remove fast_verify/trust_crc user option and never trust crc
• normalise setting names

Some possible changes are undecided:

• change "user option" terminology to "setting" (quite likely to happen)
• replace FileProgrammer with image file classes and factory function
• finish init sequence work, extending outside of CoreSightTarget, bringing session and board into the mix

This is a release-wide checklist covering MemoryMap, MemoryRangeBase, CortexM, Target, flash_loader.py, and ELF symbol classes. Start by reviewing the unchecked and undecided items, then narrow the work to one explicitly scoped API change; completion for the overall issue is not defined beyond resolving the remaining checklist items.