Add Uint/Int 256 support for Stellar (Soroban) target (#1831)

[guptapratykshh]

Description

Soroban contracts increasingly require 256-bit arithmetic compatibility with Solidity. This PR brings Solang’s Soroban backend to parity for 256-bit types, enabling broader contract portability and correctness.

Implementation -

• Encoding/Decoding

  • Split 256-bit values into four 64-bit pieces: lo_lo (0–63), lo_hi (64–127), hi_lo (128–191), hi_hi (192–255)
  • Preserve signedness during shifts for int256
  • Reconstruct from host-provided 64-bit pieces

• Host Functions

  • Declare and use new host functions:
    • U256: ObjToU256LoLo, ObjToU256LoHi, ObjToU256HiLo, ObjToU256HiHi, ObjFromU256Pieces
    • I256: ObjToI256LoLo, ObjToI256LoHi, ObjToI256HiLo, ObjToI256HiHi, ObjFromI256Pieces
  • Provide correct LLVM signatures:
    • ObjTo*: (i64) -> i64
    • ObjFrom*Pieces: (i64, i64, i64, i64) -> i64

Fixes #1831