fix str length encoding

[kgrofelnik]

Encountered an error while trying to make a transaction with a long string argument:

  File ".venv/lib/python3.11/site-packages/canoser/array_t.py", line 45, in check_value
    self.atype.check_value(item)
  File ".venv/lib/python3.11/site-packages/canoser/int_type.py", line 72, in check_value
    raise TypeError('value {} not in range {}-{}'.format(value, min, max))
TypeError: value 871 not in range 0-255

Traced the issue back to the encoding of string length.

[FrankC01]

Good catch