x/crypto/cryptobyte: add ReadUint64 and AddUint64

[AGWA]

cryptobyte.String has ReadUint8, ReadUint16, ReadUint24, and ReadUint32 but no ReadUint64.

Likewise, cryptobyte.Builder has AddUint8, AddUint16, AddUint24, and AddUint32 but no AddUint64.

I ran into the omission when writing a parser for a Certificate Transparency Merkle Tree Leaf which uses 64 bit integers for timestamps.

I propose adding:

// ReadUint64 decodes a big-endian, 64-bit value into out and advances over it.
// It reports whether the read was successful. 
func (s *String) ReadUint64(out *uint64) bool

// AddUint64 appends a big-endian, 64-bit value to the byte string.  
func (b *Builder) AddUint64(v uint64)

cc @rolandshoemaker

(edited to add AddUint64)

[rolandshoemaker]

Yup, seems reasonable.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://go.dev/cl/421014 mentions this issue: crypto/cryptobyte: add ReadUint64 and AddUint64