Reduce memory allocations for SDP marshal

[Hartigan]

Hi

I noticed a large number of memory allocations during marshal of SessionDescription. And I wanna propose implementation of fmt::Display for SessionDescription struct and some fixes fmt::Display implementations for sub types across sdp crate. Also, implementation of fmt::Display will allow to write SessionDescription into preallocated strings:
let mut out = String::with_capacity(1024); write!(out, "{sdp}");

Performance improvement according existed benchmark:
Source SDP/Marshal time: [11.610 µs 11.621 µs 11.633 µs]
New SDP/Marshal time: [4.3594 µs 4.3840 µs 4.4143 µs]

[Hartigan]

@rainliu I hope this message finds you well. Would you be able to take a moment to review this PR? Or maybe you may suggest who can take care about this PR?

[codecov]

Codecov Report

Attention: Patch coverage is 57.74648% with 30 lines in your changes missing coverage. Please review.

Project coverage is 61.31%. Comparing base (4bb9614) to head (faa999a).

Files	Patch %	Lines
sdp/src/description/session.rs	45.94%	0 Missing and 20 partials ⚠️
sdp/src/lexer/mod.rs	70.58%	0 Missing and 5 partials ⚠️
sdp/src/description/media.rs	76.92%	0 Missing and 3 partials ⚠️
sdp/src/description/common.rs	50.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #596      +/-   ##
==========================================
- Coverage   61.35%   61.31%   -0.05%     
==========================================
  Files         532      532              
  Lines       49715    49724       +9     
  Branches    12601    12660      +59     
==========================================
- Hits        30504    30487      -17     
+ Misses       9868     9865       -3     
- Partials     9343     9372      +29     

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