Add object layout, object parsing, instruction layout

* Object layout and parsing are done in the vm::obj module
* Add MemCursor, a wrapper around the std::io::Cursor type for walking
  through VM memory
* Add vm::tick module for containing the Vm::tick() method
  implementation, since it's pretty big
* Instructions are now variable-sized, and are read lazily,
  one-at-a-time directly from memory.
* Add VM runtime error structure
* Probably some other stuff I forgot

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/inst.rs

@@ -1,33 +1,39 @@
-use crate::vm::reg::*;
+macro_rules! instructions {
+    {
+        $($variant:ident = $value:expr),* $(,)?
+    } => {
+        pub type InstOp = u16;
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-#[allow(dead_code)]
-pub enum Inst {
-    Add(Reg, Reg),
-    Mul(Reg, Reg),
-    Div(Reg, Reg),
-    Mod(Reg, Reg),
-    INeg(Reg),
-    And(Reg, Reg),
-    Or(Reg, Reg),
-    Xor(Reg, Reg),
-    Shl(Reg, Reg),
-    Shr(Reg, Reg),
-
-    CmpEq(Reg, Reg),
-    CmpLt(Reg, Reg),
-    Jmp(Reg),
-    Jz(Reg),
-    Jnz(Reg),
-
-    Load(Reg, Reg),
-    Store(Reg, Reg),
-    StoreImm(Reg, u64),
-    MemCopy(Reg, Reg),
-    RegCopy(Reg, Reg),
-
-    Nop,
-    Halt,
+        $(
+            pub const $variant: InstOp = $value;
+        )*
+    };
 }
 
-// https://crates.io/crates/packed_struct
+instructions! {
+    ADD        = 0x0000,
+    MUL        = 0x0001,
+    DIV        = 0x0002,
+    MOD        = 0x0003,
+    INEG       = 0x0004,
+    AND        = 0x0005,
+    OR         = 0x0006,
+    INV        = 0x0007,
+    NOT        = 0x0008,
+    XOR        = 0x0009,
+    SHL        = 0x000a,
+    SHR        = 0x000b,
+    CMPEQ      = 0x1000,
+    CMPLT      = 0x1001,
+    JMP        = 0x1100,
+    JZ         = 0x1101,
+    JNZ        = 0x1102,
+    LOAD       = 0x2000,
+    REGCOPY    = 0x2001,
+    STOREIMM64 = 0x2100,
+    STOREIMM32 = 0x2101,
+    MEMCOPY    = 0x2200,
+    STORE      = 0x2201,
+    HALT       = 0xF000,
+    NOP        = 0xF001,
+}