Lobster: change builder size constants (enum -> let)

This is because newer versions of the Lobster compiler reject enums with the same values, and older compilers are fine with the let versions also.

lobster/flatbuffers.lobster

diff --git a/lobster/flatbuffers.lobster b/lobster/flatbuffers.lobster
index 1199e50..17a159f 100644
--- a/lobster/flatbuffers.lobster
+++ b/lobster/flatbuffers.lobster
@@ -24,15 +24,14 @@
 struct offset:
     o:int
 
-enum sizeof:
-    sz_8 = 1
-    sz_16 = 2
-    sz_32 = 4
-    sz_64 = 8
-    sz_voffset = 2
-    sz_uoffset = 4
-    sz_soffset = 4
-    sz_metadata_fields = 2
+let sz_8 = 1
+let sz_16 = 2
+let sz_32 = 4
+let sz_64 = 8
+let sz_voffset = 2
+let sz_uoffset = 4
+let sz_soffset = 4
+let sz_metadata_fields = 2
 
 class builder:
     buf = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"