Fix for parsing floats that don't include . or e

[zjijz]

Rust allows strings of integral values to be parsed to f32 or f64 using the str::parse method. I was trying to use scan_fmt to parse an integral value to a floating point, but I was getting the following panic:
Code:

let res = scan_fmt::scan_fmt!("1000", "{f}");

Error:

thread 'parse::test_integer_for_float' panicked at 'index out of bounds: the len is 4 but the index is 4', src/parse.rs:68:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The issue had to do with scan_dec10_nest potentially exhausting the input, but then scan_float called vs.cur() to check for . or e, which would throw the out-of-bounds error since we were at end-of-input. The fix for this was to check if we were at end-of-input before doing the check for . or e.

I added a test (test_integer_for_float) to check for this edge case and ensured none of the other tests failed with the change.

I also manually checked the other uses of scan_dec10_nest (and scan_dec10) and it didn't seem like it was used in a location that assumed end-of-input was not reached already.