Show signature of function pointers where possible

[Tetralux]

const std = @import("std");

pub fn main() !void {
    var a = std.debug.global_allocator;
    var bytes = a.alloc(u8, 1);
    bytes = a.reallocFn(a, bytes, 1, 2);
}

 error: expected 5 arguments, found 4
    bytes = a.reallocFn(a, bytes, 1, 2);
                       ^
path\to\zig\lib\zig\std\special\start.zig:184:40: note: referenced here
            const result = root.main() catch |err| {
                                       ^

This does not show the signature of reallocFn, which it should.
Not sure if this also happens if it's just in a different file or not.

[nektro]

note that reallocFn is a field here not a function declaration

[nektro]

its an internal value not meant to be used by consumers of the Allocator api.

the proper way to call it would be a.reallocFn(bytes, 1, 2); or instead use a.realloc(bytes, 2); as intended

[nektro]

also note Allocator.reallocFn is no longer a field in master version of Zig