Mostly putting angle brackets around links to follow markdown syntax.

* Introduce InstructionOffset for AArch64

There are a lot of instructions on AArch64 where we take an offset
from PC in terms of the number of instructions. This is for loading
a value relative to the PC or for jumping.

We were usually accepting an A64Opnd or an i32. It can get
confusing and inconsistent though because sometimes you would
divide by 4 to get the number of instructions or multiply by 4 to
get the number of bytes.

This commit adds a struct that wraps an i32 in order to keep all of
that logic in one place. It makes it much easier to read and reason
about how these offsets are getting used.

* Use b instruction when the offset fits on AArch64

* Better b.cond usage on AArch64

When we're lowering a conditional jump, we previously had a bit of
a complicated setup where we could emit a conditional jump to skip
over a jump that was the next instruction, and then write out the
destination and use a branch register.

Now instead we use the b.cond instruction if our offset fits (not
common, but not unused either) and if it doesn't we write out an
inverse condition to jump past loading the destination and
branching directly.

* Added an inverse fn for Condition (#443)

Prevents the need to pass two params and potentially reduces errors.

Co-authored-by: Jimmy Miller <jimmyhmiller@jimmys-mbp.lan>

Co-authored-by: Maxime Chevalier-Boisvert <maximechevalierb@gmail.com>
Co-authored-by: Jimmy Miller <jimmyhmiller@jimmys-mbp.lan>

There are a lot of times when encoding AArch64 instructions that we
need to represent an integer value with a custom fixed width. For
example, the offset for a B instruction is 26 bits, so we store an
i32 on the instruction struct and then mask it when we encode.

We've been doing this masking everywhere, which has worked, but
it's getting a bit copy-pasty all over the place. This commit
centralizes that logic to make sure we stay consistent.

Previously we were using a `Box<dyn FnOnce>` to support patching the
code when jumping to labels. We needed to do this because some of the
closures that were being used to patch needed to capture local variables
(on both X86 and ARM it was the type of condition for the conditional
jumps).

To get around that, we can instead use const generics since the
condition codes are always known at compile-time. This means that the
closures go from polymorphic to monomorphic, which means they can be
represented as an `fn` instead of a `Box<dyn FnOnce>`, which means they
can fall back to a plain function pointer. This simplifies the storage
of the `LabelRef` structs and should hopefully be a better default
going forward.

* LSL and LSR

* B.cond

* Move A64 files around to make more sense

* offset -> byte_offset for bcond