Specify that types are compact size unsigned ints to allow for multi-byte types

Author: achow101

bip-0174.mediawiki

@@ -106,8 +106,10 @@ The format of each key-value map is as follows:
 | Must be <tt>0x00</tt>.
 |}
 
-The first byte of each key specifies the type of the key-value pair. There are global types,
-per-input types, and per-output types.
+At the beginning of each key is a compact size unsigned integer representing the type.
+This compact size unsigned integer must be minimally encoded, i.e. if the value can be represented using one byte, it must be represented as one byte.
+For convenience, this BIP will specify types using their full serialization, so a multi-byte type will have it's full prefix and zero padding as necessary.
+There are global types, per-input types, and per-output types.
 
 The currently defined global types are as follows:
 
@@ -436,8 +438,6 @@ The Partially Signed Transaction format can be extended in the future by adding
 new types for key-value pairs. Backwards compatibilty will still be maintained as those new
 types will be ignored and passed-through by signers which do not know about them.
 
-If one byte type fields were to ever run out, new extensions can still be added by defining multi-byte types where the first byte signals that the next byte indicates the type and so on.
-
 ===Version Numbers===
 
 The Version number field exists only as a safeguard in the event that a backwards incompatible change is introduced to PSBT.