Use a sync.Pool for Unmarshal

Attributes cause the most memory pressure. Start with caching those

Before
"""
12904 B/op         48 allocs/op
"""

After
"""
7483 B/op         34 allocs/op
"""

Add marshalInto to all types

This reduces the amount of allocations need to marshal
a SessionDescription

Before
'''
5000 ns/op            2064 B/op         53 allocs/op
5050 ns/op            2064 B/op         53 allocs/op
5033 ns/op            2064 B/op         53 allocs/op
5009 ns/op            2064 B/op         53 allocs/op
4991 ns/op            2064 B/op         53 allocs/op
'''

After
'''
2237 ns/op             800 B/op          7 allocs/op
2273 ns/op             800 B/op          7 allocs/op
2271 ns/op             800 B/op          7 allocs/op
2285 ns/op             800 B/op          7 allocs/op
2265 ns/op             800 B/op          7 allocs/op
'''

Cleanup lexer syntax errors

The old message makes finding the error difficult and echos the whole
SDP.

Add attribute extmap-allow-mixed

For support with dependency descriptor, chrome will use
mixed extmap both one-byte and two-byte defined in RFC8285,
add the attribute to support that

Extmap: Add ssrc-audio-level RTP extension URI

This is for ion-sfu to be able to register this RTP extension. From
RFC6464.

Also add myself to the README as required by Metadata.Contributors CI
check.

Improve Unmarshal performance by 11%

* strings.Fields() and strconv.ParseInt() replaced by
  new functions: lexer.readField() and lexer.readUint64Field()

* Because sdp size is small, lexer now uses just simple []byte

* New parseError show invalid position

* Bug in tests messages: bytes wasn't converted to strings before output

* Bug in ConnectionInformation.String(): nil pointer dereference

Benchmarks (before/after)
BenchmarkUnmarshal-12 156142 6981 ns/op 4008 B/op 108 allocs/op
BenchmarkUnmarshal-12 167923 6193 ns/op 2584 B/op 108 allocs/op

Update CI configs to v0.4.7

Update lint scripts and CI configs.

Update CI configs to v0.4.6

Update lint scripts and CI configs.

Remove ICE logic and place into pion/ice

Remove pion/ice dependencies. This isn't SDP specific, and complicates
the dep tree for users who just want ICE

Fix session ID generation

Use crypto random according to
https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-26#section-5.2.1