enhancement for enums

[sdghchj]

Describe the PR

1. explicit enum conversion;
2. implicit integer conversion;
3. keep the type of the right operand of a shift operation; fix panic: runtime error: negative shift amount #1392 negative shift amount as a result of expression ^uint(0)
4. parse escape characters.
5. rune type in binary expressions. fix panic: runtime error: negative shift amount #1392 interface{} is nil, not int
6. function len to get the length of a literal string
7. parse hex and oct numbers

Relation issue
#1392

examples:

const uintSize = 32 << (^uint(uintptr(0)) >> 63)
const maxBase = 10 + ('z' - 'a' + 1) + ('Z' - 'A' + 1)
const shlByLen = 1 << len("aaa")
const hexnum = 0xFF
const octnum = 017
const nonescapestr = `aa\nbb\u8888cc`
const escapestr = "aa\nbb\u8888cc"
const escapechar = '\u8888'

type Class int
const A Class = 1
const B = Class(2) 

[codecov]

Codecov Report

Base: 95.04% // Head: 85.16% // Decreases project coverage by -9.88% ⚠️

Coverage data is based on head (939c8d6) compared to base (a10fb9a).
Patch coverage: 29.58% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1400      +/-   ##
==========================================
- Coverage   95.04%   85.16%   -9.89%     
==========================================
  Files          16       19       +3     
  Lines        3111     3620     +509     
==========================================
+ Hits         2957     3083     +126     
- Misses         89      459     +370     
- Partials       65       78      +13     

Impacted Files	Coverage Δ
const.go	23.20% <23.20%> (ø)
package.go	73.07% <57.69%> (-15.23%)	⬇️
utils_other.go	89.47% <89.47%> (ø)
utils_go18.go	100.00% <100.00%> (ø)

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[enricorotundo]

hey @sdghchj thanks so much for adding support for enums 🎉