Introduce description for the Register enum

In order to improve the code readability of the protect payload policy (and the rest of the codebase), this commits introduces docs comments in the Register enumeration, where each comment gives the abi name of the risc-v calling convention.

src/arch/registers.rs

@@ -4,37 +4,69 @@
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 #[repr(u8)]
 pub enum Register {
+    /// zero - Hard-wired zero register
     X0 = 0,
+    /// ra - Return address
     X1 = 1,
+    /// sp - Stack pointer
     X2 = 2,
+    /// gp - Global pointer
     X3 = 3,
+    /// tp - Thread pointer
     X4 = 4,
+    /// t0 - Temporary register
     X5 = 5,
+    /// t1 - Temporary register
     X6 = 6,
+    /// t2 - Temporary register
     X7 = 7,
+    /// s0 / fp Saved register / frame pointer
     X8 = 8,
+    /// s1 - Saved register
     X9 = 9,
+    /// a0 - Function argument / return value
     X10 = 10,
+    /// a1 - Function argument / return value
     X11 = 11,
+    /// a2 - Function argument
     X12 = 12,
+    /// a3 - Function argument
     X13 = 13,
+    /// a4 - Function argument
     X14 = 14,
+    /// a5 - Function argument
     X15 = 15,
+    /// a6 - Function argument
     X16 = 16,
+    /// a7 - Function argument
     X17 = 17,
+    /// s2 - Saved register
     X18 = 18,
+    /// s3 - Saved register
     X19 = 19,
+    /// s4 - Saved register
     X20 = 20,
+    /// s5 - Saved register
     X21 = 21,
+    /// s6 - Saved register
     X22 = 22,
+    /// s7 - Saved register
     X23 = 23,
+    /// s8 - Saved register
     X24 = 24,
+    /// s9 - Saved register
     X25 = 25,
+    /// s10 - Saved register
     X26 = 26,
+    /// s11 - Saved register
     X27 = 27,
+    /// t3 - Temporary register
     X28 = 28,
+    /// t4 - Temporary register
     X29 = 29,
+    /// t5 - Temporary register
     X30 = 30,
+    /// t6 - Temporary register
     X31 = 31,
 }