For Int32, GetHashCode just returns the value directly.

The SmallElementLocation class exists because very few element locations require 32 bits to store the line/column values. It uses ushort (2 bytes) instead of int (4 bytes) for each value, in an attempt to reduce memory usage.

However the CLR aligns ushort fields on classes at four-byte boundaries on most (all?) architectures, meaning the optimisation has no effect.

This commit explicitly packs the two values into four bytes to ensure that four bytes is saved per instance.

The caller performs this validation already, and no other code can call this. Avoid some indirection and branching.

The compiler will generate slightly better code from this switch statement, in cases where either line or column is zero.

There was inconsistent handling of validation between implementations. This moves it all into the `Create` method so that it can be handled in one place, consistently.

The CLR does in fact pack these fields adjacent to one another, so we don't have to do this in code ourselves.

Adds new subtypes for `ElementLocation` that pack to multiples of eight bytes, to avoid wasting space at runtime on padding between instances of this class in memory.

The primary gain here comes from being able to use a smaller value for the `File` value. With this change, there's a lock-free cache of file paths which are then stored by index. When the index is small, as it usually will be, it can be packed for efficiently (e.g. in 2 bytes) than a string reference (8 bytes on 64-bit architectures).

See code comment for more details.

Also remove file IO from unit tests so they run faster.