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/obj/obj.rs

@@ -0,0 +1,107 @@
+use crate::vm::obj::error::ParseError;
+use std::{
+    convert::TryFrom,
+    fmt::Debug,
+};
+
+pub const MAGIC: u64 = 0xDEAD_BEA7_BA5E_BA11;
+
+#[derive(Debug)]
+pub struct Object {
+    pub header: Header,
+    pub sections: Vec<Box<dyn Section>>,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct Header {
+    pub version: u16,
+    pub sections: u16,
+}
+
+macro_rules! section_kind {
+    (
+        pub enum $enum_name:ident {
+            $($name:ident = $value:expr),* $(,)?
+        }
+    ) => {
+
+        #[derive(Debug, Clone, Copy)]
+        #[repr(u8)]
+        pub enum $enum_name {
+            $($name = $value),*
+        }
+
+        impl TryFrom<u8> for SectionKind {
+            type Error = ParseError;
+
+            fn try_from(other: u8) -> std::result::Result<Self, Self::Error> {
+                match other {
+                    $(
+                        $value => Ok($enum_name::$name),
+                    )*
+                    _ => Err(ParseError::UnknownSectionKind { kind: other }),
+                }
+            }
+        }
+    };
+}
+
+section_kind! {
+    pub enum SectionKind {
+        Data = 0x00,
+        Code = 0x10,
+        Meta = 0xFF,
+    }
+}
+
+pub trait Section: Debug {
+    fn header(&self) -> SectionHeader;
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct SectionHeader {
+    pub kind: SectionKind,
+    pub checksum: u32,
+    pub len: u64,
+}
+
+#[derive(Debug, Clone)]
+pub struct DataSection {
+    pub header: SectionHeader,
+    pub load_location: u64,
+    pub contents: Vec<u8>,
+}
+
+impl Section for DataSection {
+    fn header(&self) -> SectionHeader {
+        self.header
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct CodeSection {
+    pub header: SectionHeader,
+    pub load_location: u64,
+    pub contents: Vec<u8>,
+}
+
+impl Section for CodeSection {
+    fn header(&self) -> SectionHeader {
+        self.header
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct MetaSection {
+    pub header: SectionHeader,
+    pub entry_count: u64,
+    pub entries: Vec<(String, Vec<u8>)>,
+}
+
+impl Section for MetaSection {
+    fn header(&self) -> SectionHeader {
+        self.header
+    }
+}
+
+