Add a new function `map` to template functions.
`map` creates a map from a list of pairs of key values, then you can
convert a key to a value using `.Get`. This simplifies writing
key-mapping templates. For example, the defaule `archives.name_template`
```yaml
name_template: >-
{{ .ProjectName }}_
{{- title .Os }}_
{{- if eq .Arch "amd64" }}x86_64
{{- else if eq .Arch "386" }}i386
{{- else }}{{ .Arch }}{{ end }}
{{- if .Arm }}v{{ .Arm }}{{ end }}
```
becomes
```yaml
name_template: >-
{{ $arch := map "amd64" "x86_64" "386" "i386" -}}
{{ .ProjectName }}_
{{- title .OS }}_
{{- $arch.Get .Arch .Arch }}
{{- if .Arm }}v{{ .Arm }}{{ end }}
```
---------
Co-authored-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>
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