Setting the value of a field of a structured box can overflow onto the next field

[wenkanglu]

Describe the bug
{box_name}.{field_name} = {value}. replaces the field {field_name} of box {box_name} created from a struct without considering the size of {value}. This issue tracks the bug when setting the value of a field to a byte array that is larger than the size defined overflowing into the next field.

To Reproduce

#pragma version 8

struct UserInfo:
    birthday: bytes[10]
    favourite_colour: bytes[20]
end

if Txn.ApplicationID == 0:
    exit(1)
end

box<UserInfo> info_box = CreateBox("foo")
info_box.birthday = "06/02/2023OVERFLOW"
exit(1)

Expected behavior
Field favourite_colour of box with key "foo" should have an empty value ("") but is instead "OVERFLOW". You could also argue that info_box.birthday = "06/02/2023OVERFLOW" should fail outright (assigning a value larger than the size defined in the struct).

Additional context
Related issue: #74

[fergalwalsh]

Discussed on Discord but want to mention here for visibility; this is being worked on as part of the current type checks work.