While looking at some mem profiles from `evm` runs, I noticed that
`goja` compilation of the bigint library was present. The bigint library
compilation happens in a package `init`, whenever the package
`eth/tracers/js` is loaded. This PR changes it to load lazily when
needed.
It becomes slightly faster with this change, and slightly less alloc:y.
Non-scientific benchmark with 100 executions:
```
time for i in {1..100}; do ./evm --code 6040 run; done;
```
current `master`:
```
real 0m6.634s
user 0m5.213s
sys 0m2.277s
```
Without compiling bigint
```
real 0m5.802s
user 0m4.191s
sys 0m1.965s
```
e4dbd5f685