Fix overly greedy digits

This fixes several rules that were using greedy repetition operators.
The problem with `(DIGIT|_)* DIGIT` is that the first repetition
consumes all of the digits.

I actually don't know exactly why the original rules were written in
this particular way. I'm guessing they were intending to express that
there must be at least one digit. However, just having `_`* should be
sufficient.

Author: ehuss

src/tokens.md

@@ -445,11 +445,11 @@ INTEGER_LITERAL ->
 
 DEC_LITERAL -> DEC_DIGIT (DEC_DIGIT|`_`)*
 
-BIN_LITERAL -> `0b` (BIN_DIGIT|`_`)* BIN_DIGIT (BIN_DIGIT|`_`)*
+BIN_LITERAL -> `0b` `_`* BIN_DIGIT (BIN_DIGIT|`_`)*
 
-OCT_LITERAL -> `0o` (OCT_DIGIT|`_`)* OCT_DIGIT (OCT_DIGIT|`_`)*
+OCT_LITERAL -> `0o` `_`* OCT_DIGIT (OCT_DIGIT|`_`)*
 
-HEX_LITERAL -> `0x` (HEX_DIGIT|`_`)* HEX_DIGIT (HEX_DIGIT|`_`)*
+HEX_LITERAL -> `0x` `_`* HEX_DIGIT (HEX_DIGIT|`_`)*
 
 BIN_DIGIT -> [`0`-`1`]
 
@@ -561,7 +561,7 @@ FLOAT_LITERAL ->
     | DEC_LITERAL (`.` DEC_LITERAL)? FLOAT_EXPONENT SUFFIX?
 
 FLOAT_EXPONENT ->
-    (`e`|`E`) (`+`|`-`)? (DEC_DIGIT|`_`)* DEC_DIGIT (DEC_DIGIT|`_`)*
+    (`e`|`E`) (`+`|`-`)? `_`* DEC_DIGIT (DEC_DIGIT|`_`)*
 ```
 
 r[lex.token.literal.float.form]