Optimize runtime sizes of some structures

[sethdusek]

ergovalidation keeps a large LRU cache of boxes to speed up validation, but this requires a large amount of memory. Reduce the size of ErgoBox. Some ideas how to do this:

• Use Box<[T]> instead of Vec<[T]> where possible. ErgoBox should be immutable anyways so growable collections are not needed
• Reduce ErgoTree size:
  • ErgoTreeHeader can be stored in 1 byte instead of 3 (don't unpack is_constant_segregation and has_size flags)
  • Use Box<[Constant]> for constants
  • Remove lazy has_deserialize. Using OnceCell requires 4 bytes of space, isn't supported on no_std and doesn't save that much time anyway (one tree pass to check if tree contains deserialize nodes). Change has_deserialize to Option, if ErgoTree was deserialized
  • Reduce Expr Size:
    • Change Spanned to use u32 indexes instead of usize
    • Expr size will be atleast the size of the largest node, which is currently MethodCall. Change MethodCall.args to Box<[Expr]>, optimize SMethodDesc similarly. In the future MethodCall could be changed to only contain information that can't be inferred so it doesn't contain the entire Method description
• Optimize Constant/SType size