loadMemoryFromFile() should place readmemh() inline for better compatibility

`loadMemoryFromFile()` creates a separate file and uses a SV bind statement. A simple example:

```
import chisel3._
import chisel3.util.experimental.loadMemoryFromFile

class Foo(val bits: Int, val size: Int, filename: String) extends Module {
val io = IO(new Bundle {
val nia = Input(UInt(bits.W))
val insn = Output(UInt(32.W))
})

val memory = Mem(size, UInt(32.W))
io.insn := memory(io.nia >> 2);
loadMemoryFromFile(memory, filename)
}

object FooObj extends App {
chisel3.Driver.execute(Array[String](), () => new Foo(32, 1024, "insns.hex"))
}
```

```
# cat Foo.Foo.memory.v
module BindsTo_0_Foo(
input clock,
input reset,
input [31:0] io_nia,
output [31:0] io_insn
);

initial begin
$readmemh("insns.hex", Foo.memory);
end
endmodule

bind Foo BindsTo_0_Foo BindsTo_0_Foo_Inst(.*);
```

Yosys doesn't like this, and likely there are other tools that don't either. Is there any reason we don't just place it inline?