Add subsidy halving interval constant

Author: casey

src/blockdata/constants.rs

@@ -61,7 +61,8 @@ pub const PUBKEY_ADDRESS_PREFIX_TEST: u8 = 111; // 0x6f
 pub const SCRIPT_ADDRESS_PREFIX_TEST: u8 = 196; // 0xc4
 /// The maximum allowed script size.
 pub const MAX_SCRIPT_ELEMENT_SIZE: usize = 520;
-
+/// How may blocks between halvings.
+pub const SUBSIDY_HALVING_INTERVAL: u32 = 210_000;
 
 /// In Bitcoind this is insanely described as ~((u256)0 >> 32)
 pub fn max_target(_: Network) -> Uint256 {