PROPOSAL: IR

This is a proposal of a custom IR to run wasm.
At the moment only `ir.zig` has some parts related
to this IR. The idea of the IR is to be a subset
of the one defined in wasm. There are some gaps
(mostly in wasm instructions that have opcode
0xFC) but in wasm they don't use all of the
opcodes for some reason so maybe we could try to
utilize them and be a superset of wasm.

src/mods/mods.zig

@@ -0,0 +1,5 @@
+pub const Parser = @import("Parser.zig");
+pub const VM = @import("vm.zig");
+// TODO: is this really needed?
+pub const Wasm = @import("wasm.zig");
+pub const IR = @import("ir.zig");