py: Store bytecode arg names in bytecode (were in own array).
This saves a lot of RAM for 2 reasons: 1. For functions that don't have default values, var args or var kw args (which is a large number of functions in the general case), the mp_obj_fun_bc_t type now fits in 1 GC block (previously needed 2 because of the extra pointer to point to the arg_names array). So this saves 16 bytes per function (32 bytes on 64-bit machines). 2. Combining separate memory regions generally saves RAM because the unused bytes at the end of the GC block are saved for 1 of the blocks (since that block doesn't exist on its own anymore). So generally this saves 8 bytes per function. Tested by importing lots of modules: - 64-bit Linux gave about an 8% RAM saving for 86k of used RAM. - pyboard gave about a 6% RAM saving for 31k of used RAM.