Purpose

Since "Cargo PR-8500" merged, as "Cargo Issue-8502" said: "if a crate will also be built as a runtime dependency, that would result in Cargo building it twice, once with opt-level 0 and once with opt-level 3."

So, in this PR I customize the opt-level to the default values, to make sure each crates only be compiled once.

Results

When make prod for v0.42.0:

• This PR can reduce the number of compiled dependencies 9.56% (596 -> 544).
• This PR can reduce the size of target directory 14.2% (1_813_100 bytes -> 1_587_780 bytes).
• Save total compile time.

Notice: the size of final binary won't be chaneged.

References

• Cargo PR-8500: Build host dependencies with opt-level 0 by default
• Cargo Issue-8502: Support opt-level ranges to avoid building twice with different opt-level values
• Cargo Reference / Default profiles