Precompute operator length for JimParseExprOperator, optimize logic

No regressions, no changes in bench.tcl.

JimParseExprOperator was shockingly inefficient, to the point of taking 63% of cpu
in my own app.

It did a linear search over >60 structs, re-computing the string length each time, and
doing a string comparison for each.

This patch pre-computes the operator string length, speeding the function up by two.

Adding the first-letter fast path and evaluating length before strncmp make for
another 2x speedup, making the total speedup 4x.

Signed-off-by: Lauri Kasanen <cand@gmx.com>
Shuffle Jim_Obj to shrink it on 64bit (72 -> 64), removing holes

No perf difference measured using bench.tcl.

Signed-off-by: Lauri Kasanen <cand@gmx.com>