[Execution state] Optimize MTrie reading a single path

[fxamacker]

Problem

Currently, MTrie.UnsafeRead reads registers in batch, so a read for a single path goes through the unnecessary steps of path partitioning and recursive calls.

Updates epic #1744 because optimization would also improve memory use.

The Proposed Solution

Create a new read function to read register for a single path. Existing read function can use the new read function to skip unnecessary steps when len(paths) == 1.

delta.View can also be optimized by using this new read function.