Add sourcemap feature

[chinedufn]

This commit introduces the "sourcemap" feature.

Users that do not need sourcemaps can disable this feature and save
roughly 15% on compile times.

Related to #357

[chinedufn]

Seeing around 15% savings in compile times on my machine when running cargo clean && cargo build --no-default-features --features grid --timings.

This PR gets rkyv out of the dependency tree for those that aren't using sourcemaps.
syn is still in there but future PRs will try to chip away at that.

Before

Around 43 seconds (consistent over a few runs)

After

Around 36 seconds (consistent over a few runs)

[chinedufn]

Conditionally compiling expressions isn't stable, so needed to wrap this in a block.

[devongovett]

I think we should probably add "sourcemap" to the default features. I guess the node bindings are getting it through "bundler" right now, though they kinda use it directly too even without the bundler.

[chinedufn]

Ok great. Just force pushed a commit with sourcemap as an explicitly default feature

[devongovett]

Thanks!