Change constant LIB to sub LIB for NativeCall.native trait

[cono]

Today I have tried to work with this module in following docker image:
jjmerelo/alpine-perl6 latest

And I got problems like this:

DBIish: DBDish::mysql needs 'mysqlclient', not found
  in block  at /root/DBIish/lib/DBIish.pm6 (DBIish) line 38
  in any  at /root/DBIish/lib/.precomp/66C92D2F17D08B89FEC541B7D4E05F5B1EE9A1EE/1C/1CA5208A6CF88323A3E082CE410A3C0FA838A19B line 1
  in method setup at /root/.rakudobrew/moar-master/install/share/perl6/sources/947BDAB9F96E0E5FCCB383124F923A6BF6F8D76B (NativeCall) line 283
  in method CALL-ME at /root/.rakudobrew/moar-master/install/share/perl6/sources/947BDAB9F96E0E5FCCB383124F923A6BF6F8D76B (NativeCall) line 570
  in method connect at /root/DBIish/lib/DBDish/mysql.pm6 (DBDish::mysql) line 12
  in method connect at /root/DBIish/lib/DBIish.pm6 (DBIish) line 45
  in block <unit> at -e line 1

I've digged this to the following boilerplate:

# perl6 -MNativeCall -e 'constant LIB = ( "mysqlclient" ); sub mysql_get_client_version(--> int32) is native(LIB) { * }; say mysql_get_client_version'
60111
# perl6 -MNativeCall -e 'constant LIB = sub { "mysqlclient" }; sub mysql_get_client_version(--> int32) is native(LIB) { * }; say mysql_get_client_version'
Cannot locate native library 'mysqlclient': mysqlclient: cannot open shared object file: No such file or directory

Discussed on #perl6 channel, and got explanatioon:

16:58 < scimon> m: constant CON = sub{ "Hmnmm"}; say CON; 
16:58 < camelia> rakudo-moar 21ae9f3e2: OUTPUT: «sub { }␤»
16:59 < scimon> m: constant CON = sub{ "Hmnmm"}; say CON.(); 
16:59 < camelia> rakudo-moar 21ae9f3e2: OUTPUT: «Hmnmm␤»

And yes, it works:

perl6 -MNativeCall -e 'constant LIB = sub { "mysqlclient" }; sub mysql_get_client_version(--> int32) is native(LIB.()) { * }; say mysql_get_client_version'
60111

Tried following workaround:

perl6 -MNativeCall -e 'sub LIB { "mysqlclient" }; sub mysql_get_client_version(--> int32) is native(LIB) { * }; say mysql_get_client_version' 
60111

So proposing this merge request to fix this problem.

[salortiz]

@cono,
Using any Callable for the is-native trait results in a call for every required symbol, with 3a4c268 the library is searched and pre-loaded if possible and LIB is constant if not.