__repr__ and __str__ have inefficient implementations

[devin-petersohn]

Describe the problem

Printing large dataframes is slow.

Also see #37. We are concatenating full columns, which is very slow for large dataframes. A solution similar to #39 will need to be implemented.

Source code / logs

import modin.pandas as pd

df = pd.read_csv(...)
print(df)

[kunalgosar]

Closed via #41