Use static memory layout for all sysvars (#249)

Co-authored-by: Richard Patel <ripatel@jumptrading.com>

sdk/program/src/epoch_rewards.rs

@@ -7,6 +7,8 @@
 //! [`sysvar::epoch_rewards`]: crate::sysvar::epoch_rewards
 
 use std::ops::AddAssign;
+
+#[repr(C)]
 #[derive(Debug, Serialize, Deserialize, PartialEq, Eq, Default, Clone, Copy, AbiExample)]
 pub struct EpochRewards {
     /// total rewards for the current epoch, in lamports

sdk/program/src/fee_calculator.rs

@@ -6,6 +6,7 @@ use {
     log::*,
 };
 
+#[repr(C)]
 #[derive(Serialize, Deserialize, Default, PartialEq, Eq, Clone, Copy, Debug, AbiExample)]
 #[serde(rename_all = "camelCase")]
 pub struct FeeCalculator {

sdk/program/src/stake_history.rs

@@ -11,6 +11,7 @@ use std::ops::Deref;
 
 pub const MAX_ENTRIES: usize = 512; // it should never take as many as 512 epochs to warm up or cool down
 
+#[repr(C)]
 #[derive(Debug, Serialize, Deserialize, PartialEq, Eq, Default, Clone, AbiExample)]
 pub struct StakeHistoryEntry {
     pub effective: u64,    // effective stake at this epoch