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In the Rust compiler, a pull request is being considered (see GitHub PR
rust-lang/rust#41336) that will add support for a += &b (instead of
only a += b) for numeric types like i64 and f64. This helps when
writing generic code.

Unfortunately, this breaks the build of bank, because bank
currently does the following in a few places in ogm.rs:

data += input[4..8].parse().map_err(|_| "Quad is not numerical")?;

Rust is currently able to infer that parse() really means
parse::<u64>(), because the result eventually gets added to a u64.
But if the PR on the compiler is accepted, this type inference will no
longer be possible, because parse() could also be parse::<&'a u64>()
for some lifetime 'a.

This commit specifies the output type of parse() explicitly,
so that it doesn't require any type inference.