[ENH] io: Speedup write_data

[ales-erjavec]

Issue

Speed write_data.

Description of changes

Assemble column formatters beforehand, avoid inline logic deciding the
correct format for every cell value.

Includes

• Code changes
• Tests
• Documentation

[ales-erjavec]

In [1]: import Orange
In [2]: adult = Orange.data.Table("adult")
In [3]: %timeit adult.save("test-save-temp.tab")

before:
1.07 s ± 7.55 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
after:
386 ms ± 5.88 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

[BlazZupan]

This is a great speed-up. I have tested it on data with over 20,000 features and 100 data instances, where speed-up was 6-fold.