expose Go embed.FS of schemas and vectors

[cpu]

This allows downstream Go code to import this repo and directly access JSON schema files and JSON test vector files using a FS interface (listing files, reading content, etc).

We already have a go.mod to manage the Go vectorlint program. I believe this addition meets the language specific ecosystem considerations. I will be relying on it downstream and commit to helping keep it working.

Example usage:

package main

import (
	"fmt"
	"io/fs"

	"github.com/c2sp/wycheproof"
)

func main() {
	fmt.Println("schemas")
	schemas, _ := fs.ReadDir(wycheproof.Schemas, ".")
	for _, entry := range schemas {
		fmt.Printf("\t%s\n", entry.Name())
	}

	testvectors, _ := fs.ReadDir(wycheproof.TestVectors, ".")
	fmt.Println()
	fmt.Println("testvectors_v1/")
	for _, entry := range testvectors {
		fmt.Printf("\t%s\n", entry.Name())
	}
}

[botovq]

There are better people among the reviewers to evaluate go here, but this looks indeed handy.

[cpu]

@FiloSottile flipped through this while we were on a call, suggested some review feedback:

1. Add just one .go file with both embeds to minimize cluttering up the root
2. Avoid the stutter with the field name + fs root dir (e.g. wycheproof.Schemas.ReadDir("schemas") needing "schemas")

I'll address both items shortly.

[cpu]

I'll address both items shortly.

Done, and the usage example in the PR description was updated accordingly. I'll merge this in the next few days unless there's any new feedback.