Backup from_base_le strategy for very large Uints

[prestwich]

At some size of Uint, the recursion in from_base_le will cause stack overflows even for valid iterator sizes. This does not happen in from_base_be, as it does not recurse for each iterator item

• Set a safe maximum size for the recursive from_base_le
• Provide a backup strategy that is used only above that size.
  • This strategy may use alloc. If so, return an error if the alloc feature is not enable.