Fix potential name collisions in code generated by the table! macro

[weiznich]

This commit fixes potential name collisions in code generated by the
table! macro by prefixing function argument names with
__diesel_internal. The underlying problem here is that rust accepts
patterns as part of the function signature. This combined with the fact
that each zero sized struct (so all column/table structs) are type +
value at the same time results in problems as soon as the users tries to
create a column/table with the same name as one of the used arguments.
So for example if the users has a table that contains an alias column,
that would have caused a name collision inside of the following function
signature:

fn map(column: C, alias: &$crate::query_source::Alias<S>) -> Self::Out

Rustc would have interpreted alias here as zero sized struct literal,
which of course has a different type than the corresponding RHS. That
results in a compiler error about mismatched types.

This commit fixes this problem by prefixing the corresponding variables
with __diesel_internal, which is hopefully reasonably uncommon, so
that no one uses that as column name prefix.