Parse header compact forms

[kognise]

RFC 3261 Section 7.3.3 Compact Form specifies that several headers have single-character compact names, and:

Implementations MUST accept both the long and short forms of each header name.

A total of 10 compact forms are defined by the specification:

Header	Compact Form
Call-ID	i
Contact	m
Content-Encoding	e
Content-Length	l
Content-Type	c
From	f
Subject	s
Supported	k
To	t
Via	v

This PR adds support for parsing them. Should be an easy review :)

Thanks for the great crate!

[hkrutzer]

FYI the library author writes here that it is being developed in a closed source manner now.

[kognise]

thanks! that's a shame but i hope it's open sourced eventually!