Binary format: Don't emit empty Memory sections

[kripken]

If the only memories are imported, we don't need the section. We were already
doing that for tables, functions, etc.

The binary testcase here is this (in a binary that has no Memory section):

(module
 (type $none_=>_i32 (func (result i32)))
 (import "a" "b" (memory $mimport$0 0))
 (export "test" (func $0))
 (func $0 (result i32)
  (i32.load
   (i32.const 10)
  )
 )
)

Noticed here: paritytech/substrate#12280 (comment)