This PR adds high level support for Boxes with a new box<struct> type and some functions for creating/opening boxes.

Where Item is a previously defined struct:

box<Item> item1 = CreateBox("a") # asserts box does not already exist
box<Item> item1 = OpenBox("a")   # asserts box does already exist and has the correct size for the struct
box<Item> item1 = Box("a")       # makes no assertions about the box

Example:

struct Item:
    id: int
    foo: int
    name: bytes[10]
end

box<Item> item1 = CreateBox("a")

item1.id = 1
item1.foo = 111
item1.name = "xyz"

log(itob(item1.foo))

See the included example for more details.

Box field assignments use box_replace while box field access uses box_extract to ensure support for large boxes - the total box size can be up to the 32K bytes limit but individual fields can be up to 4096 to allow them to fit on the stack. Future enhancements to Tealish may add support for arrays and nested structs. It would then be possible to have larger fields if only portions were assigned/accessed.

Note: This PR also includes an update to the langspec.json to sync with go-algorand/master including avm8 opcodes.