Making FromDigits more like StringContext; FromDigits literal suffixes

[s5bug]

Right now, FromDigits doesn't have any way to change the type of a literal without ascribing it. Ideally, one could get custom literals typed correctly via suffixes, such as 1_234bi for a way to acquire BigInts using underscores, and so on.

[bishabosha]

suffixes could work, except that identifiers would be limited by how they could be ambiguous when mixed with hexadecimal digits, I considered once using some kind of separator, like 0x7f'b for a byte, which I implemented in this commit

[som-snytt]

interesting idea!

[soronpo]

Since we are dealing with terms, perhaps we can use # in prefix position:
b#0x7f
bi#1234_5678_8910_1213_1415_16_17
ip#127.0.0.1
So maybe we do not need FromDigits at all, but just a special case for string interpolation syntax to make it simpler for numerical expressions. This way bi#1234_5678_8910_1213_1415_16_17 will be equivalent to bi"1234_5678_8910_1213_1415_16_17".

[soronpo]

@odersky I'm interested in your thoughts about this proposal.