Avoid creating Strings during distribution gen (WIP)

[alamb]

• Related to [FEATURE] Reduce latency to create first row ( initial table generation time) #56

I was trying to make the startup time faster and one thing I figured I could do is stop copying strings during text pool generation.

However I made the change to avoid copying and it doesn't really seem to have helped, but I figured I would put up the PR in case someone else was interested

Main

(venv) andrewlamb@Andrews-MacBook-Pro-2:~/Software/tpchgen-rs$ time target/release/tpchgen-cli -s 1 --tables=nation --output-dir=/tmp/tpchdbgen-rs
Generation complete!

real	0m1.955s
user	0m1.901s
sys	0m0.051s

This branch:

branch 'alamb/faster_table_gen' set up to track 'alamb/alamb/faster_table_gen'.
(venv) andrewlamb@Andrews-MacBook-Pro-2:~/Software/tpchgen-rs$ time target/release/tpchgen-cli -s 1 --tables=nation --output-dir=/tmp/tpchdbgen-rs
Generation complete!

real	0m1.953s
user	0m1.910s
sys	0m0.039s

[alamb]

the idea was to not allocate strings

[alamb]

This was done for real in #63