32-bit SWAR for string escaping

[streamich]

For JsonEncoder implement SWAR (SIMD within a register) for detecting escaping 4 characters at a time:

// Checks if characters need escaping in a packed input (4 bytes in uint32_t).
constexpr bool NeedsEscape(uint32_t input) {
  constexpr uint32_t mask_0x20 = 0x20202020u;
  constexpr uint32_t mask_0x22 = 0x22222222u;
  constexpr uint32_t mask_0x5c = 0x5C5C5C5Cu;
  constexpr uint32_t mask_0x01 = 0x01010101u;
  constexpr uint32_t mask_msb = 0x80808080u;
  // Escape control characters (< 0x20).
  const uint32_t has_lt_0x20 = input - mask_0x20;
  // Escape double quotation mark (0x22).
  const uint32_t has_0x22 = (input ^ mask_0x22) - mask_0x01;
  // Escape backslash (0x5C).
  const uint32_t has_0x5c = (input ^ mask_0x5c) - mask_0x01;
  // Chars >= 0x7F don't need escaping.
  const uint32_t result_mask = ~input & mask_msb;
  const uint32_t result = ((has_lt_0x20 | has_0x22 | has_0x5c) & result_mask);
  return result != 0;
}