feat: support no_std

[CPunisher]

closes #12

1. bytes and simdutf8 support for no_std makes it possible for faststr to support no_std.

2. According to https://doc.rust-lang.org/cargo/reference/features.html#dependency-features:

   The "package-name/feature-name" syntax will also enable package-name if it is an optional dependency. Often this is not what you want. You can add a ? as in "package-name?/feature-name" which will only enable the given feature if something else enables the optional dependency.

   Changing serde/std to serde?/std decreases the library size from 159kb to 142kb on aarch64.

[CLAassistant]

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[PureWhiteWu]

Thanks very much for your contribution!

Could you please add an CI accourding to https://www.lurklurk.org/effective-rust/no-std.html#writing-code-for-no_std ?

[PureWhiteWu]

Thanks again for your contribution!