[stdlib] Add implicit decorators to constructors, NFC

This adds `@implicit` to all single-argument constructors to enable
explicit by default construction. Implicit conversions are used heavily
throughout the codebase, and so this is a required step to get it to
compile. This allows us to take off `@implicit` constructors where
they don't make sense, and fix the compiler errors one step at
a time.

MODULAR_ORIG_COMMIT_REV_ID: 931a2f38a0f9fe87007e334158f1fdf399f04bb2

Signed-off-by: Manuel Saelices <msaelices@gmail.com>

stdlib/src/builtin/object.mojo

@@ -755,6 +755,7 @@ struct object(
     # FIXME: None literal should be of NoneType not !kgen.none.
     @doc_private
     @always_inline
+    @implicit
     fn __init__(out self, none: __mlir_type.`!kgen.none`):
         """Initializes a none value object from a `None` literal.
 

stdlib/test/builtin/test_none.mojo

@@ -47,6 +47,7 @@ struct FromNone:
     fn __init__(out self, none: __mlir_type.`!kgen.none`):
         self = NoneType()
 
+    @implicit
     fn __init__(out self, value: Int):
         self.value = value