Notes - Add MIR doc (rough outline)

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+% Mid-level intermediate representation
+
+
+See https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
+
+
+Overview
+========
+
+Graph of "Basic Blocks"
+- Stored as a vector, integer indexes between each other
+- All function calls terminate their block
+- Varaibles are single-assigment, but mutable via &mut or field accesses
+
+
+Types
+=====
+
+LValues (assignable locations)
+------------------------------
+
+- `B` - User-declared binding (with a name and slot index)
+- `TEMP` - Compiler-inserted temporary (with just an index)
+- `ARG` - Function argument (with index only)
+- `STATIC` - Reference to a `static` or `static mut`
+- `RETURN` - The function's return value
+- `LVALUE.f` - Refers to the field `f` of the inner `LVALUE`
+- `*LVALUE` - Dereference a pointer (&-ptr or rawptr)
+- `LVALUE[LVALUE]` - Index into an array (slice accesses use operator overloads)
+- `(LVALUE as VARIANT)` - Downcast `LVALUE` into the specified enum variant (to access it as that variant)
+
+RValues (usable values)
+-----------------------
+
+- `use(LVALUE)` - Read out of an LVALUE
+- `&'rgn LVALUE` - Immutably borrow an LVALUE
+- `&'rgn mut LVALUE` - Mutably borrow an LVALUE
+- `LVALUE as TYPE` - Primitive cast operation
+- `LVALUE <op> LVALUE` - Binary operation (on numeric primitives only)
+- `<op> LVALUE` - Unary operation (numeric primitives and booleans only)
+- `[LVALUE; LVALUE]` - Construct a sized array
+- `[LVALUE, ...]` - Construct a literal array
+- `(LVALUE, ...)` - Construct a tuple
+- `PATH { f: LVALUE, ... }` - Construct a named struct
+- `meta(LVALUE)` - Extract the fat pointer metadata for an lvalue
+- `fatptr(LVALUE, LVALUE)` - Construct a fat pointer from a pair of lvalues
+- `CONSTANT` - Subset of RValues that are known at compile-time
+
+