[WIP] Fix assumption on int size in ecmult_wnaf function

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Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>_ecmult_wnaf relies on int having at least 32 value bits</issue_title>
<issue_description>The current secp256k1_ecmult_wnaf needs the unstated and unchecked assumption that int has at least 32 value bits when it VERIFY_CHECKs that w <= 31. In practice, we call it only with WINDOW_A == 5 and WINDOW_G == ECMULT_WINDOW_SIZE where the latter is configurable in the range 2..24.

A consequence of this "bug" is that the code fails on a 16-bit platform if you set ECMULT_WINDOW_SIZE > 16. I don't think we need to support this, but code without unchecked assumptions is bad. So I suggest that we rewrite the function to use int32_t instead of int even if we don't use my macro approach. (We can keep the "bit position" arguments and variables int.)

Alternatively, we could add the assumption that INT_MAX >= INT32_MAX but this forbids 16-bit platforms, and the code seems to work on them in principle; see #792 (comment).

Originally posted by @real-or-random in #1761 (review)
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<agent_instructions>Use the suggestion to use int32_t instead of int. Keep the "bit positions" variables and arguments int.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes _ecmult_wnaf relies on int having at least 32 value bits #1769

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