lj_str.c: Remove special-case string interning fastpath
lj_str.c: Remove special-case string interning fastpath

lj_str_new() had a separate fast-path and slow-path. This was bad
because (a) the fast-path was complex and (b) the fast-path was
actually slower than the slow-path in practice and (c) in practice it
could cause confusing performance problems depending on the memory
alignment of any often-reused string buffers in a program.

This change specifically makes the 'life' benchmark faster and more
robust to memory layout.

eugeneia/snabb,Igalia/snabbswitch,Igalia/snabb,eugeneia/snabb,Igalia/snabb,Igalia/snabb,Igalia/snabbswitch,snabbco/snabb,snabbco/snabb,Igalia/snabb,Igalia/snabb,eugeneia/snabb,Igalia/snabb,SnabbCo/snabbswitch,alexandergall/snabbswitch,alexandergall/snabbswitch,snabbco/snabb,eugeneia/snabb,eugeneia/snabb,snabbco/snabb,SnabbCo/snabbswitch,Igalia/snabb,alexandergall/snabbswitch,eugeneia/snabb,alexandergall/snabbswitch,Igalia/snabbswitch,alexandergall/snabbswitch,SnabbCo/snabbswitch,snabbco/snabb,snabbco/snabb,eugeneia/snabb,Igalia/snabbswitch,Igalia/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,Igalia/snabb,eugeneia/snabb,alexandergall/snabbswitch,snabbco/snabb,SnabbCo/snabbswitch,alexandergall/snabbswitch