Unambiguous char-to-number and char-to-digit conversions

[qurbonzoda]

This issue is to discuss the proposal to introduce new Char <-> Int conversion API to make conversions unambiguous.
The new API includes:

fun Char(code: Int): Char
val Char.code: Int
fun Char.digitToInt(radix: Int = 10): Int
fun Char.digitToIntOrNull(radix: Int = 10): Int?
fun Int.digitToChar(radix: Int = 10): Char

The proposal text is here: https://github.com/Kotlin/KEEP/blob/master/proposals/stdlib/char-int-conversions.md.

[ilya-g]

Alternative placement suggestion:

• Char(code) function and Char.code property are so closely related to Char type that they could be placed into kotlin package, or even become members of the Char type itself.

[ilya-g]

Alternative API shape suggestion:

• instead of providing default value for radix, the functions digitToInt, digitToIntOrNull, digitToChar could have overloads with and without radix, similar to String.toInt and String.toIntOrNull. The motivation is the same: the implementation with the constant radix equal to 10 can be made more efficient.

[qurbonzoda]

The suggestions from @ilya-g were discussed and accepted. The KEEP document was updated.