Replace String with SmolStr in Plan

To measure performance boost, I use following benchmarks: cloning plan, serializing plan, and deserializing as well as other benchmarks from sbroad benches.
As a query to build plan from I took this query. After replacing String with SmolStr, we get following results:

	String, µs	SmolStr, µs
clone	100.50	61.161
serialize	56.428	58.167
deserialize	252.16	251.32
merge_several_column_statistics	25.129	25.568
pure_pest_parsing_many_references	394.76	389.53
pure_pest_parsing_target_query0	24.270	24.589
pure_pest_parsing_target_query1	32.364	32.827
pure_pest_parsing_target_query2	49.337	50.044
full_parsing_many_references	961.74	1000,43
full_parsing_target_query0	93.369	83.511
full_parsing_target_query1	110.84	100.21
full_parsing_target_query2	165.82	160.82

As a side effect, it also speeds up full_parsing_* benchmarks by 8-9%.

Documentation follow-up: docs#220 (closed)