perf: improve default case of equal constraint by decoding in bulk

[jesusvazquez]

Michael's idea to improve an inefficient Get() access for non ascending primary columns

Before we were doing

for j := bl; j < br; j++ {
    if !ec.matches(symbols.Get(j)) {  // Individual lookup per row
        // ...
    }
}

Now we're doing

symbols.DecodeBuffer(buf, bl, br) // Single bulk dictionary lookup
for j := bl; j < br; j++ {
    if !ec.matches(buf[j-bl]) { // Direct array access
        // ...
    }
}

[jesusvazquez]

Moved this up since its now needed for both cases