Support `no_std` build

[bavshin-f5]

Since we have our own allocation, string and IO primitives, there's almost nothing we need from the std:: namespace in the module SDK itself. no_std build would reduce the module footprint and help weeding out remaining heap allocations.

It is of course acceptable to use std:: for the nginx modules, but that should be opt-in with a clear understanding of drawbacks and benefits.