exportSince: (1, 5, 1): export when nimversion >= (1,5,1)

[timotheecour]

Example

proc fn*(a: int) = discard
proc fn(a: float) {.exportSince: (1,5,1).} = discard

behavior: this exports a symbol when nimversion >= (1,5,1)

Note: the pragma should only affect the symbol for which the pragma applies, which is tricky in the case of overloads.

implementation

using this PR nim-lang#12076 we can write:
resolveSymbol(fn(float.default)) to pick a particular overload, and then a macro to export that particular overload (a symbol) can be written.

links

exportSince
https://github.com/nim-lang/Nim/pull/13044/files#r368188265

see also nim-lang#16044 (comment)

since has a serious design flaw, it should only disable the export marker when used with --useVersion so that it can be used regardless in the same module. I know it doesn't help stdlib clients of the module but it would help. This probably means that since should come a compiler builtin...