Support memarg load/store instructions

[xtuc]

The syntax is described here: https://webassembly.github.io/spec/core/syntax/instructions.html#syntax-memarg.

WAST grammar (spec):

offset: offset=<nat>
align: align=(1|2|4|8|...)
<type>.load((8|16|32)_<sign>)? <offset>? <align>?
<type>.store(8|16|32)? <offset>? <align>?

Example:

(i64.load offset=0x000 align=1 (i32.const 0))