Replace exists with explicit nil check

[jakebailey]

This function was useful when we were using interfaces and didn't want to care about boxing nil, but now we're all concrete so I don't think we need this function anymore.

This saves a good 5% in bind.

goos: linux
goarch: amd64
pkg: github.com/microsoft/typescript-go/internal/compiler
cpu: Intel(R) Core(TM) i9-10900K CPU @ 3.70GHz
         │   old.txt   │              new.txt               │
         │   sec/op    │   sec/op     vs base               │
Bind-20    24.58m ± 1%   23.45m ± 2%  -4.58% (p=0.000 n=10)
Parse-20   45.97m ± 1%   46.25m ± 3%       ~ (p=0.063 n=10)
geomean    33.61m        32.93m       -2.01%

         │   old.txt    │               new.txt               │
         │     B/op     │     B/op      vs base               │
Bind-20    9.776Mi ± 0%   9.776Mi ± 0%       ~ (p=0.912 n=10)
Parse-20   23.59Mi ± 0%   23.59Mi ± 0%       ~ (p=0.684 n=10)
geomean    15.19Mi        15.19Mi       +0.00%

         │   old.txt   │              new.txt               │
         │  allocs/op  │  allocs/op   vs base               │
Bind-20    102.6k ± 0%   102.6k ± 0%       ~ (p=0.670 n=10)
Parse-20   239.1k ± 0%   239.1k ± 0%       ~ (p=0.896 n=10)
geomean    156.6k        156.6k       -0.00%