der: nested reader depth limit may reject the documented maximum depth of 64

[dmulder]

The nested-reader depth limiting added in der 0.8.1 appears to have a possible off-by-one discrepancy.

src/reader/position.rs uses a check equivalent to:

depth < MAX_DEPTH

with MAX_DEPTH set to 64. However, the accompanying comments or tests appear to describe 64 levels as the supported maximum. Depending on how the current depth is counted, the strict < comparison may reject the level described as the maximum.