Support reading/writing LEF via definitions

[rahulk29]

Adds support for VIA definitions as described in the LEF 5.8/6.0 specification:

• "Fixed" vias (ie. those with an explicitly listed set of shapes)
• "Generated" vias (ie. those described by a VIARULE)

Note that although we support parsing both kinds of vias, we don't yet support parsing VIARULE GENERATE statements (which generated vias reference).

[dan-fritchman]

Alright! Welcome back.
PR looks great.
A heads up there are a few others open for LEF parsing & writing - namely #53 #54 #56.
CC @nanobowers

[rahulk29]

Thanks :)
Happy to take another look at this PR once the other PRs have been merged.

[rahulk29]

@dan-fritchman Is there anything blocking merging those PRs?

[dan-fritchman]

PRs #53, #54, and #56 are now merged. Thanks @nanobowers !
@rahulk29 can you roll those changes into your own?

[rahulk29]

@dan-fritchman Done!

[rahulk29]

@dan-fritchman merge if this is good to go?

[dan-fritchman]

MERGE MERGE MERGE