Upgrade to Syn 0.14

[Rantanen]

Figured we'd need this at some point anyway and I wanted the custom_keyword! macro for parsing attribute parameters.

Also fun times moving from Ident to &Ident again after going the other way to satisfy Clippy a while back. \o/

(Most of the changes are simple Ident <-> &Ident changes.)

[codecov-io]

Codecov Report

Merging #76 into master will decrease coverage by 0.52%. The diff coverage is 70%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
- Coverage   74.56%   74.04%   -0.53%     
==========================================
  Files          56       56              
  Lines        3436     3467      +31     
==========================================
+ Hits         2562     2567       +5     
- Misses        874      900      +26

Impacted Files	Coverage Δ
intercom-common/src/lib.rs	100% <ø> (ø)	:arrow_up:
intercom-common/src/builtin_model.rs	91.3% <ø> (ø)	:arrow_up:
intercom-common/src/attributes/com_library.rs	0% <0%> (ø)	:arrow_up:
intercom-common/src/methodinfo.rs	86.36% <100%> (ø)	:arrow_up:
intercom-common/src/attributes/com_impl.rs	98.24% <100%> (ø)	:arrow_up:
intercom-common/src/attributes/com_class.rs	100% <100%> (ø)	:arrow_up:
intercom-common/src/returnhandlers.rs	88.37% <100%> (ø)	:arrow_up:
intercom-common/src/model.rs	89.17% <100%> (-0.16%)	:arrow_down:
intercom-common/src/foreign_ty.rs	92.85% <100%> (-7.15%)	:arrow_down:
intercom-common/src/attributes/common.rs	100% <100%> (ø)	:arrow_up:
... and 13 more

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