Add dynlib.initialize_symbols (#3071)

```
package example

import "core:dynlib"
import "core:fmt"

Symbols :: struct {
	// `foo_` is prefixed, so we look for the symbol `foo_add`.
	add: proc "c" (int, int) -> int,
	// We use the tag here to override the symbol to look for, namely `bar_sub`.
	sub: proc "c" (int, int) -> int `dynlib:"bar_sub"`,

	// Exported global (if exporting an i32, the type must be ^i32 because the symbol is a pointer to the export.)
	// If it's not a pointer or procedure type, we'll skip the struct field.
	hellope: ^i32,

	// Handle to free library.
	// We can have more than one of these so we can match symbols for more than one DLL with one struct.
	_my_lib_handle: dynlib.Library,
}

main :: proc() {
	sym: Symbols

	// Load symbols from `lib.dll` into Symbols struct.
	// Each struct field is prefixed with `foo_` before lookup in the DLL's symbol table.
	// The library's Handle (to unload) will be stored in `sym._my_lib_handle`. This way you can load multiple DLLs in one struct.
	count := dynlib.initialize_symbols(&sym, "lib.dll", "foo_", "_my_lib_handle")
	defer dynlib.unload_library(sym._my_lib_handle)
	fmt.printf("%v symbols loaded from lib.dll (%p).\n", count, sym._my_lib_handle)

	if count > 0 {
		fmt.println("42 + 42 =", sym.add(42, 42))
		fmt.println("84 - 13 =", sym.sub(84, 13))
		fmt.println("hellope =", sym.hellope^)
	}
}
```

core/dynlib/doc.odin

@@ -1,7 +1,11 @@
+//+build ignore
 /*
 Package core:dynlib implements loading of shared libraries/DLLs and their symbols.
 
 The behaviour of dynamically loaded libraries is specific to the target platform of the program.
 For in depth detail on the underlying behaviour please refer to your target platform's documentation.
+
+See `example` directory for an example library exporting 3 symbols and a host program loading them automatically
+by defining a symbol table struct.
 */
-package dynlib
+package dynlib