Make sure that the key argument on Get doesn't need to be allocated on heap

[cipriancraciun]

Currently, when one calls cdb.Get(key), if the caller has the key on stack, like key := [4]byte {0,1,2,3}, the Go compiler decides that the value "escapes" and thus it must be allocated on the heap. (The reason being that cdb.hash uses that key value, and an arbitrary implementation might decide to keep a slice of the key.)

This can be spotted with the following simple test:

func BenchmarkStackEscapeYes(b *testing.B) {
    db, err := cdb.OpenMmap("./test/test.cdb")
    require.NoError(b, err)
    require.NotNil(b, db)

    for i := 0; i < b.N; i++ {
        keyOnStack := [2]byte {'X', byte (i)}
        keySlice := keyOnStack[:]
        db.Get(keySlice)
    }
}

If comipilled as go test -gcflags -m one can see the message ./noescape_test.go:32:3: moved to heap: keyOnStack.

The following two patches solve this issue:

• cipriancraciun@5b54da1 -- adds the NoEscapeBytes function based on what Go's runtime does internally -- https://github.com/golang/go/blob/ecb2f231fa41b581319505139f8d5ac779763bee/src/runtime/stubs.go#L172-L181
• cipriancraciun@e860a79 -- a simple patch of the Get function that uses the NoEscapeBytes, which tricks the Go compiler into not requiring the key to be heap allocated;

The performance improvements are about ~10ns per call, which if also using the mmap patch sent earlier represents 50% of the total runtime.

BenchmarkStackEscapeNo-4        1000000000               9.368 ns/op           0 B/op          0 allocs/op
BenchmarkStackEscapeYes-4       1000000000              23.12 ns/op            2 B/op          1 allocs/op