Merge MapTable implementation. FossilOrigin-Name: a76088fdadf8020e5727288a8ed15e9c11b69ae2b50246629ca0b87ef3c0042c
Add initial files based on previous GitHub repository. FossilOrigin-Name: 26493bcf2de9c6d35abd6f854f584773f31eb29f1806ff28926f5428d4b0ff27