Supporting Intrinsic `LOAD` and `STORE` 64 bit values about porth

[rexim]

@zrthxn this looks good for now. In the future I plan to approach the size of load/store slightly differently, but for now it's alright.

[rexim]

@zrthxn nah, .64 and ,64 are alright. I don't really care much about specific names since they always can be changed later while we are in active development phase and nothing is stabilized yet.

[zrthxn]

So for now I've tested with just 64 bit LOAD and STORE. The operators are rather unimaginative, ,64 and .64

// Test 1000...0001 (64 bit)
mem 1 63 shl 
1 + .64
mem ,64 print

// Test UINT64_MAX
mem 18446744073709551615 .64 
mem ,64 print

[zrthxn]

@rexim Do you want to use different operators...? Maybe .64 and ,64 aren't the best

[zrthxn]

Okay then #54 is waiting approval :)