Fix lint errors and warnings.

arithmetic_integers: remove >100 char line.
hailstone: remove >100 char line and minor nits.
lzw: Remove ~str, remove >100 char line and fixed spacing.
parallel_calculations: remove trailing spaces, minor simplifications.
markov_algorithm: remove >100 char line, remove ~str.

Second commit has additional cleanup and `~str` removal from all but `anagrams.rs`.
a+b: Simplify.
ackermann_function: remove to_str().
fasta: remove ~str.
integer_sequence: remove to_str().
rot13: remove ~str.