Ensure that char doesn't box when using generic math

[tannergooding]

These APIs weren't explicitly defined, so they deferred down to the DIM which can box on some runtimes or scenarios.

I went and checked what other DIMs weren't explicitly redefined and found the newer IUtf8SpanParsable APIs weren't handled on char, Int128/UInt128, BigInteger, or Complex. Additionally IUtf8SpanFormattable wasn't handled on BigInteger. I'll handle these in a different PR

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[stephentoub]

which can box on some runtimes or scenarios.

Could you share an example where boxing was occurring?

[tannergooding]

Could you share an example where boxing was occurring?

My mistake, the boxing was from a different bit of code in the sample I was looking at.

This one is fine since its a static method and therefore only ever deals with the TSelf. It's only the instance DIMs that are problematic since this is INumberBase<TSelf>

[stephentoub]

Phew, ok, thanks :)