Function zcash_deserialize_string_external_count

pub fn zcash_deserialize_string_external_count<R: Read>(
    external_byte_count: usize,
    reader: R,
) -> Result<String, SerializationError>

zcash_deserialize_external_count, specialised for String. The external count is in bytes. (Not UTF-8 characters.)

This allows us to optimize the inner loop into a single call to read_exact().

This function has a zcash_ prefix to alert the reader that the serialization in use is consensus-critical serialization, rather than some other kind of serialization.