Stats safer encode

[max-hoffman]

We previously used commas as serialization boundaries for multi-field stats tuples (bucket bounds). That worked well for numeric values, and doesn't work well for strings with commas. This uses the prolly serialization code to more safely round trip tuples.

We still use commas to separate MCV counts, which are integers, and newlines (\n) for index types. If types can have newlines at some point we would want to switch that to prolly encoding as well.

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
9746326	ok	5937457

version	total_tests
9746326	5937457

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
6ce6230	ok	5937457

version	total_tests
6ce6230	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
4a7e8ac	ok	5937457

version	total_tests
4a7e8ac	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
8437aa2	ok	5937457

version	total_tests
8437aa2	5937457

correctness_percentage
100.0