Nicer error message on name duplicates

[dahlia]

Continued from #254. Now give nicer error message on name duplicates. Offset line/column number became accurate.

Where the following Nirum code is given:

union bar = foo
          | bar
          | baz;

Although the compiler had given the following error message (which refers to inaccurate offset line/column number):

examples/a.nrm:5:1:
the facial type name `bar` is duplicated


^

It's now like the following:

examples/a.nrm:3:13:
the tag name `bar` is duplicated

          | bar
            ^

See also the added "handling name duplications" test case.

[checkmate-bot]

Checklist 🤔

src/Nirum/Parser.hs

• If a new reserved keyword is introduced, it has to be also added to reservedKeywords set in the Nirum.Constructs.Identifier module.

[codecov]

Codecov Report

Merging #255 into master will increase coverage by 0.64%.
The diff coverage is 94.02%.

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
+ Coverage   73.13%   73.77%   +0.64%     
==========================================
  Files          33       33              
  Lines        2356     2391      +35     
  Branches      130      124       -6     
==========================================
+ Hits         1723     1764      +41     
  Misses        503      503              
+ Partials      130      124       -6

Impacted Files	Coverage Δ
src/Nirum/Parser.hs	86.86% <94.02%> (+2.42%)	⬆️

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