Add support for nativeToScVal to convert strings to all number sizes.

[Shaptic]

Previously, i32 and u32 could not automatically come from string values when using nativeToScVal. Namely, nativeToScVal("4", { type: "i32" }) was explicitly forbidden. This was my design: the rationale was that if it's a 32-bit integer, it should be passed as a Number.

However, this logic does not hold true for maps, because in JavaScript, all map keys are strings. Therefore, creating an ScVal representing an integer-to-integer map is impossible via nativeToScVal, e.g., a function signature like this one:

fn spin(env: Env, pairs: Map<u32, u64>) -> boolean;

cannot be executed by

nativeToScVal(
  { 1234: 5678 }, 
  { 
    type: { 
      "1234": [ "u32, "u64" ],
      "5678": [ "u32, "u64" ],
    }
);

because 1234 would be interpreted as a string, and thus could not be automatically converted to a number for a u32 inference.

[github-actions]

Size Change: +1.48 kB (+0.05%)

Total Size: 3.21 MB

Filename	Size	Change
dist/stellar-base.js	2.36 MB	+1.08 kB (+0.05%)
dist/stellar-base.min.js	857 kB	+400 B (+0.05%)

_{compressed-size-action}

[Shaptic]

Reviewers' note: this is just whitespace changes from a stray yarn fmt (not sure how the unformatted versions snuck in)

[sreuland]

detailed pr description was very helpful for doing review, thanks!

[willemneal]

Good catch!