Use symbols for mtg link and symbol

[VEZY]

Implementing #74

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	dccd05a...	master / dccd05a...
mstg_linux/many_queries/ancestors_repeated	0.982 ± 0.1 s	1.03 ± 0.11 s	0.95 ± 0.14
mstg_linux/many_queries/ancestors_repeated_inplace	0.277 ± 0.024 s	0.261 ± 0.032 s	1.06 ± 0.16
mstg_linux/many_queries/children_repeated	0.0718 ± 0.00014 ms	0.0768 ± 0.00012 ms	0.936 ± 0.0023
mstg_linux/many_queries/parent_repeated	0.1 ± 0.00059 ms	0.0976 ± 0.00011 ms	1.03 ± 0.0062
mstg_linux/traverse/full_tree_nodes	1.9 ± 0.19 ms	1.16 ± 0.19 ms	1.63 ± 0.32
mstg_linux/traverse_extract/descendants_mass	5.44 ± 0.3 ms	5.4 ± 0.28 ms	1.01 ± 0.077
mstg_linux/traverse_extract/descendants_mass_inplace_1	0.0498 ± 0.0074 ms	0.05 ± 0.0086 ms	0.997 ± 0.23
mstg_linux/traverse_extract/descendants_mass_inplace_2	5.55 ± 0.38 ms	5.29 ± 0.25 ms	1.05 ± 0.087
time_to_load	2.2 ± 0.016 s	2.21 ± 0.0031 s	0.998 ± 0.0074

Memory benchmarks

	master	dccd05a...	master / dccd05a...
mstg_linux/many_queries/ancestors_repeated	13.1 M allocs: 0.586 GB	13.1 M allocs: 0.586 GB	1
mstg_linux/many_queries/ancestors_repeated_inplace	5.24 M allocs: 0.156 GB	5.24 M allocs: 0.156 GB	1
mstg_linux/many_queries/children_repeated	0 allocs: 0 B	0 allocs: 0 B
mstg_linux/many_queries/parent_repeated	0 allocs: 0 B	0 allocs: 0 B
mstg_linux/traverse/full_tree_nodes	10 allocs: 1.69 kB	10 allocs: 1.69 kB	1
mstg_linux/traverse_extract/descendants_mass	27 allocs: 1.68 MB	27 allocs: 1.68 MB	1
mstg_linux/traverse_extract/descendants_mass_inplace_1	0.043 k allocs: 0.669 MB	0.043 k allocs: 0.669 MB	1
mstg_linux/traverse_extract/descendants_mass_inplace_2	23 allocs: 1.68 MB	23 allocs: 1.68 MB	1
time_to_load	0.149 k allocs: 11.2 kB	0.149 k allocs: 11.2 kB	1