Allow `block size` to be an independent variable

[rec]

You should be able to set zero, one or two of these variables, and have the rest pop out with good defaults:

• block_size
• version
• ECC

It's trickier than it seems:

1. The block_size/chunk_size conversion also depends on two external variables, qr.Writer.document_bytes and qr.Writer.index_bytes

2. Not all pairs are legal - for example, if you set a high block_size and a low version, there will be no ECC that will actually work.