Pre-allocate and reuse value-path for decoding

[turbolent]

Description

Just like in #858 for the encoder, pre-allocate the path and re-use it in the decoder.

Also, improve the naming a bit to avoid accidental confusion of the various "indices" that are declared and used for different purposes.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[fxamacker]

Looks good! 👍