fix: support compilation to wasip1

[achille-roussel]

Problem

I'm interested in getting ristretto to build to the new wasip1 target, but it fails due to ristretto's assumption that the program is being compiled to a unix system where files can be mapped to memory. At this time, the x/sys/unix package does not consider wasip1 to be a unix platform, and memory mapping isn't supported either when compiling to WebAssembly.

$ GOOS=wasip1 GOARCH=wasm gotip build
package github.com/dgraph-io/ristretto
	imports github.com/dgraph-io/ristretto/z
	imports golang.org/x/sys/unix: build constraints exclude all Go files in /go/pkg/mod/golang.org/x/sys@v0.0.0-20221010170243-090e33056c14/unix

Solution

The proposed solution in this PR is to provide stubs implementations of the memory mapping functions which return ENOSYS to indicate that the functionalities are not available when compiled to GOOS=wasip1.

If I'm reading the code correctly, this means that persisting data structures to a storage medium will not be supported, but the rest of ristretto should work as expected.

I believe this to be a useful trade-off since many dependents of ristretto only use the in-memory data structures (e.g. https://github.com/DataDog/datadog-agent/blob/main/pkg/obfuscate/cache.go, which is what I'm interested in getting to compile).

A more advanced solution could be to rework the z.Buffer internals so the data structure can work on platforms where mapping files to memory isn't supported; however, this represents a bigger undertaking, I would like to first unblock dependencies by allowing the program to compile.

Please let me know if you have any concerns or feedback on the change!

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[mangalaman93]

Thank you for the PR