Remove column names from DataFrame model

[kozlov-alexey]

Motivation: before this change column names were passed to DF ctor as arguments of LiteralString types (each name of it's own type), which seems to add to linear dependency of LLVM IR size and hence impact DF ctor compile time. Since this information is saved into DF type itself and can be captured in any of DF methods on typing it's proposed to remove columns from DF model struct as redundant.

[kozlov-alexey]

Some numbers on reducing IR/compilation time:

n_columns		8	16	32	64	128	256	512
LLVM IR size, B	on master	379255	723548	1596652	4081771	12016803	39670140	142028921
LLVM IR size, B	With PR #944	301594	580848	1324210	3547908	10947168	37538424	137766111
reduced by, %		20.48	19.72	17.06	13.08	8.90	5.37	3.00
compilation time, s	on master	0.50796127	0.45004487	0.93897152	1.86501479	5.34555006	20.642166	128.379645
LLVM IR size, B	With PR #944	0.42104888	0.36896634	0.63800144	1.33405638	3.82808781	16.295801	116.300309
reduced by, %		17.11	18.02	32.05	28.47	28.39	21.06	9.41