Large stack memory usage

[occheung]

Description

With a large MTU / RAW_FRAME_LENGTH_MAX at 0x1000, large amount stack space is consumed. Seems that large arrays are allocated onto the stack on transmitting/receiving packets.
On transmitting case, these few lines of code may have contributed to this issue.

ENC424J600/src/tx.rs

Lines 54 to 60 in 010be3e

	impl TxPacket {
	pub fn new() -> Self {
	TxPacket {
	frame: [0; RAW_FRAME_LENGTH_MAX],
	frame_length: 0
	}
	}

ENC424J600/src/lib.rs

Lines 161 to 163 in 010be3e

	// Copy packet data to SRAM Buffer
	// 1-byte Opcode is included
	let mut txdat_buf: [u8; RAW_FRAME_LENGTH_MAX + 1] = [0; RAW_FRAME_LENGTH_MAX + 1];

Shrinking RAW_FRAME_LENGTH_MAX to 1500 have improved this significantly in my case.
Is it worth looking into reusing large arrays? Or even allow static buffers to be supplied by user?