Reduce Immix block size to 8 KB

Details

A block size of 8 KB reduces memory usage when running many processes, at the cost of a tiny increase in garbage collection timings. This increase is likely something we can optimise in the future. Reducing the block size also appears to speed up the test suite by 5-10 milliseconds.

Corresponding issue

Fixes #159 (closed)

Checklist

• Added tests
• Added documentation
• Inko source code follows the Inko style guide