Cleanup implicit conversions

[jackkoenig]

Remove public vals from implicit classes that can surprisingly be
invoked yet do nothing. Many implicit classes were Value Classes which
require the argument to be a public val but the avoidance of runtime
objects compared to the rest of Chisel runtime is minimal at best.

Related issue: NA

Type of change: bug fix

Impact: API modification

This PR is not source-code compatible since someone could be accidentally invoking one of these "methods". We could instead do this via a deprecation schedule.

Development Phase: implementation

Release Notes

Removed implicitly added identity methods .data and .target from Data and DecoupledIO, and removed .x from String, Int, Long, BigInt, and Boolean

[ucbjrl]

lgtm - deferring to @ducky64 for approval

[ducky64]

lgtm

Not sure why those were vals, especially since none of the common tutorials on implicits have them as vals...

[jackkoenig]

So they do have to be vals if it's a Value Class (the ones that extended AnyVal), but the only purpose of Value Classes is to provide an implicit class without runtime boxing. I thought about changing those vals to be self or _self or something but since such runtime boxing is a drop in the bucket of the overfall performance of Chisel, I thought it cleaner just to convert them to standard implicit classes and removes the vals.