Deferred epoch initialization

[kvark]

Fixes #29
Our create function is pretty lightweight now: checking a few conditions and adding a single reference. No locking or other heavy operations.

Performance before:

     Running target/release/deps/ecs_pos_vel-7c237e2b0cc65da2
test bench_build  ... bench:     851,896 ns/iter (+/- 140,450)
test bench_update ... bench:       9,533 ns/iter (+/- 1,672)
     Running target/release/deps/sc_graph-db013b6502f79d2b
test bench_build  ... bench:     486,189 ns/iter (+/- 17,393)
test bench_update ... bench:       1,895 ns/iter (+/- 176)

After:

     Running target/release/deps/ecs_pos_vel-7c237e2b0cc65da2
test bench_build  ... bench:     585,123 ns/iter (+/- 122,336)
test bench_update ... bench:       9,046 ns/iter (+/- 919)
     Running target/release/deps/sc_graph-db013b6502f79d2b
test bench_build  ... bench:     447,402 ns/iter (+/- 52,071)
test bench_update ... bench:       2,287 ns/iter (+/- 282)

Looks like a solid win (1.5x on ecs_pos_vel).

[vitvakatu]

typo

[vitvakatu]

inline?

[kvark]

Thanks, fixed!