Faster ChiselPlugin

[jackkoenig]

Use caching to reduce number of expensive lookups. Also make methods private so we can improve them without breaking binary compatibility

I think this could be better, like only doing a single lookup, but I was having trouble getting it working so thought it better to just get this out here. This reduces the runtime of the Chisel plugin in rocket-chip compile by quite a bit (collected with scalacOption -Ystatistics):

Before:
[info]   typer                       : 1 spans, ()32910.11ms (27.8%)
[info]   chiselcomponent             : 1 spans, ()29308.0ms (24.7%)

After:
[info]   typer                       : 1 spans, ()26130.235ms (42.1%)
[info]   chiselcomponent             : 1 spans, ()6523.072ms (10.5%)

Typer included for reference (also shows how noisy the measurements are).
With this change, chiselcomponent is generally 2-4x faster.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• [NA] Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• performance improvement

API Impact

Some methods that shouldn't have been public in the compiler plugin are now private

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Improve performance of the Chisel Scalac compiler plugin over that in 3.4.0-RC1

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (3.2.x, 3.3.x, 3.4.0, 3.5.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?