Improve writing performances

[ragoune]

We can remove the entitiesNames map in the World structure. This relational map was used to keep track of the entities names per their EntityId.

The EntityId being a uint64, we can simply generate this using a hash value of the EntityName instead of a random: it simplifies the code, reduces the time spent to generate the random id (the hash seems faster than the rand).
Moreover, removing this relational map hugely reduces the memory usage (from 57996246 to 41214216 Bytes/operation in the benchmark BenchmarkCreateEntityVolt).

It was already the case but not strictly documented: the entity name MUST be unique, otherwise we would only rewrite on the same entity.

[codecov]

Codecov Report

Attention: Patch coverage is 76.66667% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
world.go	76.66%	5 Missing and 2 partials ⚠️

Files with missing lines	Coverage Δ
world.go	71.33% <76.66%> (-5.33%)	⬇️

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