writer: split embedded data into chunks

[tuxcanfly]

Fixes #5

TODO:

• Update Read to re-group chunks
• Test that >520B data is chunked
• Test that data is re-grouped back in Read

[tuxcanfly]

Just realized this will fail if canonical size is >2 because we're using a fixed size:

			// script template:
			//                                           < ---------          35 bytes        --------->
			// OP_FALSE OP_IF + "roll" marker +  <data> + canonical int + 32 bytes pubkey + OP_CHECKSIG
			//                   ^                      ^
			// start ------------                       ^
			// end -------------------------------------

We'll be slicing off a byte from the last. @Ferret-san I think we should just use a end marker, like "kit" or something. I can then just do end := bytes.Index(witness, END_MARKER)

[tuxcanfly]

Just realized this will fail if canonical size is >2 because we're using a fixed size:

			// script template:
			//                                           < ---------          35 bytes        --------->
			// OP_FALSE OP_IF + "roll" marker +  <data> + canonical int + 32 bytes pubkey + OP_CHECKSIG
			//                   ^                      ^
			// start ------------                       ^
			// end -------------------------------------

We'll be slicing off a byte from the last. @Ferret-san I think we should just use a end marker, like "kit" or something. I can then just do end := bytes.Index(witness, END_MARKER)

Nevermind, this works. Because the last canonical int is size of pubkey which is fixed.