Uac2 fixes

* DescriptorLength is unused and therefore removed
* InputTerminalDescriptor has OverloadControl and no ClockFrequencyControl
  according to Table 4-9 of [Audio20]

Create CODE_OF_CONDUCT.md

Rework USB Audio Class stuff

* Split UAC from MIDI
* Split UAC into UAC1, UAC2, UAC3 as descriptors and
  other constants are often defined differently

Minimal change to allow support of HID descriptors

descriptors: pass descriptor collection to ConfigurationDescriptorEmitter

This allows for downstream descriptor emitters to add indexed strings.

fixes #15

Implement useable subset of USB Audio Class 2 (Audio + MIDI)

rename construct.py to fix unittest imports

descriptors: add types.descriptors.partial for parsing potentially incomplete data

This re-allows parsing partial descriptors, which had previously been allowed until
7672fb5221.

a0ceb18c89 broke this.

types.descriptors: handle Audio 1.0 endpoint descriptors

emitters: allow endpoint-less configurations

Fix DeviceDescriptorCollection always adding a language descriptor

fix BCD fields encoding

For some case the encode function was raising AssertionError despite
the BCD value beeing correct (in this case it was 1.11).

There is two main modification in the version I propose:

First the added rounding before converting the "percent" to an integer.
This may seems odd but it is actually to "correct" the error introduce
by the multiplication.

Second the comparison is done "after" conversion by recreating the
floating point value from the integer and percent variables. This is to
make sure that the value in the descriptor is the same as intended.

I've made sure that all correct values (that can be stored in bcd fields)
doesn't raise an error when passed to the encode function.

I've used the following program to test the new implementation:
```
for obj in [float(f"{i:02}.{p:02}") for i in range(100) for p in range(100)]:
    integer = int(obj) % 100
    percent = int(round(obj * 100)) % 100
    if float(f"{integer:02}.{percent:02}") != obj:
        print (f"BCD conversion error: {integer:02}.{percent:02} != {obj}")
```