cql3: Add term::to_string()
cql3: Add term::to_string()

There is some code which tries to print a term. Some terms are
printable, but not all of them. Those who don't override to_string()
will get the default implementation.

scylladb/scylla,wildinto/scylla,tempbottle/scylla,rluta/scylla,dwdm/scylla,avikivity/scylla,gwicke/scylla,bowlofstew/scylla,eklitzke/scylla,respu/scylla,capturePointer/scylla,victorbriz/scylla,gwicke/scylla,glommer/scylla,rentongzhang/scylla,stamhe/scylla,scylladb/scylla,asias/scylla,linearregression/scylla,respu/scylla,raphaelsc/scylla,dwdm/scylla,justintung/scylla,tempbottle/scylla,justintung/scylla,acbellini/scylla,victorbriz/scylla,dwdm/scylla,glommer/scylla,wildinto/scylla,kangkot/scylla,victorbriz/scylla,shaunstanislaus/scylla,aruanruan/scylla,raphaelsc/scylla,linearregression/scylla,kangkot/scylla,duarten/scylla,rentongzhang/scylla,phonkee/scylla,stamhe/scylla,senseb/scylla,rentongzhang/scylla,phonkee/scylla,kangkot/scylla,rluta/scylla,bowlofstew/scylla,kjniemi/scylla,wildinto/scylla,guiquanz/scylla,shaunstanislaus/scylla,avikivity/scylla,asias/scylla,capturePointer/scylla,guiquanz/scylla,senseb/scylla,avikivity/scylla,justintung/scylla,stamhe/scylla,gwicke/scylla,shaunstanislaus/scylla,guiquanz/scylla,acbellini/scylla,scylladb/scylla,glommer/scylla,asias/scylla,acbellini/scylla,phonkee/scylla,duarten/scylla,raphaelsc/scylla,respu/scylla,rluta/scylla,capturePointer/scylla,kjniemi/scylla,linearregression/scylla,scylladb/scylla,aruanruan/scylla,eklitzke/scylla,eklitzke/scylla,duarten/scylla,aruanruan/scylla,senseb/scylla,tempbottle/scylla,bowlofstew/scylla,kjniemi/scylla