add get_opcode and its test, fix a couple bugs + semantics things

twasm/README.md

@@ -144,6 +144,20 @@ type metadata for the different types is as follows:
 11b ; 64 bit
 ```
 
+#### `opcodes.by_id`
+
+entries are as follows:
+
+```
++----------+--------+----------+
+| 31    24 | 23  16 | 15     0 |
++----------+--------+----------+
+| reserved | opcode | token ID |
++----------+--------+----------+
+```
+
+note the lack of support for multiple-byte opcodes or multiple opcodes for one token ID; these features will likely be added at some point after the parser accumulates too much jank.
+
 ### token IDs
 
 supported tokens are listed below