[lang] Add cpp impl of ArgPackType

[listerily]

Stack from ghstack (oldest at bottom):

• [lang] [ir] Support argpack nesting #8273
• [llvm] [ir] Implemented argpack buffer load on llvm devices #8267
• [lang] Argpacks stores scalar values only in cpp implementation, and passes ndarray values by 'virtual parameters' #8266
• [test] Recover skipped argpack tests (spir-v backends only) #8265
• [Bug] Fix argpack nesting issues #8264
• [spirv] [ir] Support argpack buffer load for spir-v backends #8263
• [Bug] Convert matrices to structs in argpack type members, Fixing layout error #8262
• [lang] Add arg_depth property to Expressions and Statements #8259
• [lang] Add is_argpack property to Argument and pass argpack ptrs to LaunchContextBuilder #8257
• [bug] Fix indexing crash in TaskCodeGenLLVM::get_struct_arg #8256
• [lang] Add set_arg methods to ArgPack and pass scalar values to ArgPack buffer #8255
• [lang] Instantiate a runtime ArgPack object when a python ArgPack is created #8241
• [refactor] Renamed allocate_memory_ndarray to allocate_memory_on_device #8240
• [lang] Link cpp ArgPackType object to python ArgPackType #8178
• -> [lang] Add cpp impl of ArgPackType #8176
• [refactor] Enhance argument data structures, replacing vector styled arguments with nestable styled arguments #8216
• [refactor] Make arg_id a vector #8204

[lin-hitonami]

LGTM