Replacing the value of a field of a structured box can keep parts of the replaced value

[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 parts of the replaced value remain after replacing it with a byte array of a smaller size than the current value.

To Reproduce

#pragma version 8

struct UserInfo:
    favourite_colour: bytes[20]
end

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

box<UserInfo> info_box = CreateBox("foo")
info_box.favourite_colour = "yellow"
info_box.favourite_colour = "red"
exit(1)

Expected behavior
Field favourite_colour of box with key "foo" should have a value of "red" but is instead "redlow".

Additional context
Related issue: #75

[fergalwalsh]

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